There's a lot of reasons kinda rolled into the term. It's kinda like with what I said before with how some people use the term shipping, or maybe replace it with pairings or whatever:
It all comes down to that "what if", especially when people get invested into a character or characters.
Romance is definitely the dominant ideal now for "shipping". But that's just one type of relationship. With other pairings, there's "what if these characters were best friends" or "what if these characters became family" or "what if these characters were enemies" and so on.
I've had people ship my characters before, or hell -- they've made AUs for them if they was only one possibly romance in the story. It just boils down to really caring about the character and investing time into them. Which means there are different ways to do that.
Some people just draw fanart of the same characters, not really caring about pairings. Some people create entire headcannons for characters, and some just write the fanfics.
It all comes down to it being a form of fandom, and like any part of fandom, it can look a bit weird, if not toxic, when a loud minority makes it so. But eh -- what else is new?