It serves as an argument as to what sort of requirements might be reasonable to ask for in order to exercise a constitutional right.
If asking for ID to buy a gun is reasonable, then the argument of whether it’s reasonable to ask for ID to vote can’t be one of principle, but has to be one of degree.
The degree seems pretty large, though, since it's the difference between an unclearly worded constitutional amendment that does not in any plain reading say that "buying a gun" is a constitutional right, to literally the right to have a say in how your nation is run.
I think of it this way: If you think that you're being unconstitutionally prevented from buying a gun because you don't have an ID you can at least vote for people who would change that. If you can't vote because you don't have an ID, though, you're SOL.
Edit: The second amendment even refers to "well regulated", which to me seems to imply that the intent was to, well, have some regulation of the matter.
The meaning of words changes with time, but that can't be allowed to change our laws. Here, "well regulated" is about being reliable, high-performing, and accurate. The term also applies to mechanical devices; one could have a well regulated clock.
If asking for ID to buy a gun is reasonable, then the argument of whether it’s reasonable to ask for ID to vote can’t be one of principle, but has to be one of degree.