Descartes' analysis in the third meditation which equated the power for creating and preserving existence the same is important for how he sees God in relation to him as a cause. I think that this moment in Descartes at the same time shows the distinction between two different kinds of causes, and then conceals them once again. These causes begin to be unraveled again (in the canon passed down) by Leibniz, and the distinction is made again in Kant.
As a result of the second meditation, Descartes has placed the reader, and himself, before his own simple cognitions. When you remove all inferences, and attempts to judge, Descartes still finds we have the raw activity of cognition - the seeming appearings, the loving and hating, the fact of judging (even if the truth is doubtable). These shimmerings constitute our existence and this is what is referred to by the famous cogito sum.
The third meditation works on this more or less pure state recovered in the second meditation and attempts to analyze it to discover what else can be found in it. When Descartes considers the cause of existence to be the same as the cause of the preservation of existence, we can get a clear picture of what he means by looking at our experience, which contains as a matter of fact a continuity as part of what is in every cognition. How does Descartes take this experience, which contains a temporal depth (in its internal reference to the having been of the previous moment) and decide that it is the same thing to have this continuity as to have the simple existence? Before I try to speak to this let me make some remarks about its apparent affect on history after him.
In Descartes' inheritors the results of the collapsed distinction between existence and preservation (continuity) are fascinating. In Malebranche we find that at each moment God must act to set everything in its place (occasionalism). In Spinoza we find something even more interesting which reveals a lot about what must happen when these things are collapsed: it no longer makes sense to say that the ego is a substance underlying the sequence, but that we must immediately have the absolute substance (God) be the unity of the continuity of events. In both Malebranche and Spinoza we have God operating at every 'moment': everything is a miracle.
Lurking in the continuity of the cognitions is the 'great principle' put forward by Leibniz: the Principle of Sufficient Reason. Through this principle, Leibniz will differentiate between the series of events (which is infinite) and the cause of the entire series itself (for Descartes, these were preservation and existence). Leibniz found it intolerable that God would be performing a miracle at every moment, as apparently had to be the case under Descartes' analysis. Additionally, the production of the 'I' (ego) in Leibniz is understood on the basis of the continuity of cognitions: in the Monadology, Leibniz discusses how apperception, as that which unified perceptions, is what produces the 'I' through a reflective process. With differentiating between the continuity and the existence of moments of the series, Leibniz has introduced the substance of the subject again (which allows him to have more substances than just God, as in Spinoza).
Kant, perhaps without knowing it, takes Descartes' result in the second meditation more seriously, and along with the insights off Leibniz, continues more or less where Descartes has left off with an analysis of what is contained in the continuity of the experience. What Descartes collapsed into the same cause, is for Kant's resolution to the third antinomy, the distinction between natural causes and spontaneous causes. This distinction, in Kant, is the basis of the division of reason into the theoretical and the practical. Many other implications in Malebranch and Spinoza, as well as things uncovered again by Leibniz and Kant, could be added to this, but I will save this for another time.
Returning to Descartes, what is it that he was seeing that allowed him to collapse the distinction that he had made between existence and preservation (continuity)? It seems, to me, that one could see the continuity of the sequence as something that is entirely contained in the moment we are cognizing, and that we have no reason to refer to the existence of another time before (the moment is entirely self-contained). In Leibniz we find that the element in the moment which refers back transcends the moment and require just as much existence in the previous state in order to bring about the current one such that one original creation is sufficient for the series to continue. At this point we have two speculative claims - that only this moment exists and that the whole series must exist. In both Descartes and Leibniz the cause of existence is thought in terms of something in addition to the cognition of an object as such.
In Kant, the connection of the sequence is a rule of the understanding, and existence is not something we add to the object (as an extra judgment), but as part of the structure of cognition generally. In Kant we do not find any reason to make the speculative claim that there is something like an 'efficient cause' of existence, or continuity, as seemed demanded by Descartes and Leibniz.
I think it is important to note that the advance that Kant seemed to make in his analysis is all possible in the picture that even Descartes had provided, and it would be interesting to try to carry the clarification through the rest of Descartes analysis (as well as those of other thinkers before and after). When Descartes and Leibniz turned from their analysis - perhaps too soon - and started deriving knowledge from their conclusions, they turned to a scientific procedure which they hadn't eventuated yet in terms of its possibility.
