What you did, where you went, that thing they said they have coming up, the names of their kids, big things in their kids future, birthdays, bands they said they liked, places they said you should visit, where they live, where they have lived, institutions they've worked at, why their granddad is notable, minor celebrities they're related to, areas of science they're interested in, games they said they'd like to play.

Stuff that was part of the conversation that you might want to recall next time you're speaking to them, or that might make you contact them.

If you know your co-worker's child is named "Sam" you don't want to say "son" if she's actually a girl.

If you don't know your co-worker's child's gender, or know it's neither "male" or "female", you don't want to be forced to choose.

Why not? What if you have children and get invited to their birthday party? You would want to choose appropriate presents and greeting cards. It's a tool to help you keeping track of information you don't want to keep track of in your head. My coworkers children are the perfect example for that. Something I do not really care for but something that would make a huge difference.