I ask because I was there recently for a story mission to boost my defib, but an NPC in Wellspring told me that something was going on there and it was worth going back. Is this something that I should do just for the sake of going back? Or is it part of a later mission? Just wondering...

The game makes you go back there sooner or later storywise. There are areas you can't explore on your first walkthrough.

okay so the game will send me back there, and I don't just have to randomly go back on my own?

Make sure you grab the Large Mutant card when you go back to Dead City. You cannot go back a 3rd time if you miss it.