What's the purpose of adding a campaign when enrolling a membership
I don't have this use case and am not sure how you want to engage your constituents with this information linkage, but one possible use is in reporting. You can filter on campaigns, so if you want a list of members that have a differentiator not based on amount or date of membership, then you might assign them to specific campaigns. Similarly, "campaign" can be selected in Advanced search and used to create a smart group that can be used for mailings.