The love interest is the superhero's primary link to a world that doesn't include beating up clown terrorists and aliens. They normalize the hero, without making the hero's feats seem any less shit-your-pants awe-inspiring. They need to be more self aware than the hero, and more equipped to handle the side of the world that the superhero's secret identity doesn't handle so well.
Our biggest problem with this trope is that when you have a giant dude thing in a cape, it's easy for screenwriters to focus on that and ignore the love interest entirely. So you get a ton of character development that stops with "Here she is. She's a scientist. That means she's smart. Okay, back to the falling buildings."
The Movie That Failed At This: Spider-Man 3
Dad, he's in a band, and he has a sweet trucker hat collection.
Gwen Stacy is introduced to give the story a little "Will they or won't they?", but it's hard to accomplish that when your character descriptors are "Has a face" and "Not Mary Jane Watson."
The Movie That Did It Best: Superman
There is no part of this movie that isn't adorable. Even Gene Hackman.
"You've got me? Who's got you?!?" says a lot of what you need to know about Lois Lane in this series. She's funny, energetic, inquisitive, and everything that Superman isn't. It flips over the old "Oh, MY HERO" trope of women getting saved by burly men in colorful fetish wear and makes it into something that isn't so 1934. Most importantly, it places them as equals. Sure, Superman can seemingly invent a new power whenever he wants to, but Lois Lane will call him on his (and everyone else's) shit for the rest of their lives.