Although calculus is not frequently used in discrete mathematics it is nice to know that the students have had at least some exposure to sets and functions. I am teaching discrete this summer and find myself saying "you have seen this in calculus" when talking about several fundamental concepts.

When doing proofs in a calculus course I usually try to point out the fundamental concepts from the course that are needed and in a discrete course the actual process of how do do a proof is studied more closely. Again it is nice to know that at least the students have seen proofs before and we can build on this exposure.