In his correspondence with (a then very youthful) Joseph Butler, Samuel Clarke writes the following:
Necessity absolute and antecedent in the order of nature to the existence of any subject has nothing to limit it; but if it operates at all (as must needs do), it must operate (if I may so speak) everywhere and at all times alike…. (Clarke’s “Answer to Butler’s Third Letter,” Dec 10, 1713)
To put Clarke’s point informally: absolute necessity has the same impact everywhere and all times and should have the same consequence everywhere and all the time. So, if such necessity operates in some respect, Y, then we ought to expect Y to be homogenous in relevant ways. And if the most fundamental form of necessity operates in all (possible) respects, then we ought to expect general homogeneity.
This view of necessity turns out to be a non-trivial principle that, surprisingly enough, is shared by Clarke and Spinoza, and, even in considerable part, by Newton. (Recall Newton's claim in the General Scholium (added to the third edition): “No variation of things arises from blind metaphysical necessity, which must be the same always and everywhere.”) I emphasize two features of Clarke’s position. First, according to Clarke necessity is the “formal cause” of God (Clarke’s “answer to sixth Letter to another Gentleman” [published first in 1738]). That is to say, Clarke does not just allow the question, ‘what causes God?’ to be intelligible, but it even has a proper response: in a certain sense, one can say that for Clarke God is the consequence of a species of necessity. There is, in fact, an unmistakable echo of this doctrine of the relationship between necessity and God in Newton's "General Scholium:" “’Tis allowed by all that the supreme God exists necessarily; and by the same necessity he exists  always and every where” (emphasis added). In the quote, Newton insists (i) that God’s necessary existence is common ground among all philosophers. Newton, then, (ii) ascribes to necessity explanatory ‘power’ to help account for (at least some of) God’s properties. It’s this type of explanatory power that is captured by Clarke’s claim that necessity is a formal cause.
Second, Clarke uses the necessity as formal cause of God principle to rule out the existence of multiple substances (Clarke’s first response to Butler, 10 nov 1713, and see Clarke's A Demonstration of the Being and Attributes of God, [hereafter Demonstration] VII). So, like Spinoza [(E1D1 & E1p7)] & (E1p5&E1p7), Clarke treats God as the only true substance. In particular, in order to make credible substance monism, Clarke relies on the idea that absolute necessity does not just have the same impact everywhere and all times and should have the same consequence everywhere and all the time, but also that in the most absolute sense these consequences ‘displace’ the ‘instantiation’ of other hypothetical (but not genuinely possible) effects derived from hypothetical (but not genuinely possible) causes. 
If we attend to some of the further details of Clarke’s letter to Butler, we also learn something about the explicit target of Clarke’s argument:
Necessity absolute and antecedent in the order of nature to the existence of any subject has nothing to limit it; but if it operates at all (as must needs do), it must operate (if I may so speak) everywhere and at all times alike….
The argument is likewise the same in the question about the origin of motion. Motion cannot be necessarily existing because, it being evident that all determinations are equally possible in themselves, the original determination of the motion of any particular body this way rather than the contrary way could not be necessary in itself, but was either caused by the will of an intelligent and free agent, or else was an effect produced and determined without any cause at all, which is an express contradiction: as I have shown in my Demonstration, p. 14. (Clarke’s Answer to Butler’s Third Letter, Dec 10, 1713)
In this quoted passage Clarke relies on a version of the principle that if necessity operates in some respect, Y, then we ought to expect Y to be homogenous in relevant ways; and he applies this necessity principle to the case of motion in order to show that the origin of motion cannot be explained by necessity (in the way that God is so caused). Rather than pointing to the empirical existence of variety (as Newton does in the "General Scholium," recall: “No variation of things arises from blind metaphysical necessity, which must be the same always and everywhere,”) in the letter to Butler, Clarke argues that there is nothing about matter and/or motion as such that inclines it in one way rather than other ways.
One way to understand Clarke’s claim is that a particular directionality is a contingent fact about matter. There are different metaphysical positions compatible with Clarke’s claim here, but we can put his main point a bit metaphorically as follows: at the origin of the universe (or at any other given time and space [recall that necessity works everywhere and all times in the same way]), a particle does not ‘know’ in what direction it has to be oriented, so that the very ‘first’ motion and its determinate direction has to be non-necessary and this entails either (a) that there was a willing God or (b) no cause at all. But (b) is impossible because it denies a version of the PSR (the so-called causal principle), so, therefore, there must be (a) a willing God. Clarke tacitly rules out here the possibility of a brute, contingent fact (because it conflicts with the PSR). (In a Demonstration, Clarke repeats and elaborates on the argument with more care.)
It turns out that P. 14 of Clarke’s Demonstration is devoted to refuting “Mr. Toland’s pernicious opinion of motion being essential to matter. ” There is a marginal reference to Toland’s (1704) Letters to Serena. The subtitle of A Demonstration reads “More Particularly in Answer to Mr. Hobbs, Spinoza and Their Followers.” Toland is he only “follower” (of either Hobbes or Spinoza) named in A Demonstration. In particular, he attributes to Toland the idea that a ‘willing’ God is not required to explain the origin of motion.
 I am quoting from Clarke, S. (1998). A Demonstration of the Being and Attributes of God and other Writings. Ezio Vailati (ed). Cambridge: Cambridge University Press.
 For a very profound and pioneering treatment of the role of formal causation in Spinoza’s metaphysics, see K. Hübner “On the significance of formal causes in Spinoza’s metaphysics."
 This is not the place, alas, to explore why it would be obvious to Newton that one cannot take a genuine atheist seriously.
 One might wonder how Newton avoids being a full-blooded Spinozist. But note that Newton never explicitly embraces the second feature that I attribute to Clarke and Spinoza.
 On some fairly standard matter theories (e.g., Aristotelian and Epicurean), bits of matter do have a particular directionality (‘down’ or ‘toward the center of the universe’) built into or added to their natures.) In his correspondence with Bentley, Newton is at pains to disassociate his theory from such theories.