The point is that someone wanted to make it, and then made it. It doesn’t have to have billions of users to have been fun to make, or even to have reasons for existing. Pixelfed needs a few people to want to use it to keep existing. Instagram needs all of you + your data. And once it inevitably does something people are unhappy with, Pixelfed will be there. And once the service people will somehow find themselves on after Instagram does the same, Pixelfed will probably still be there, then. As long as one person is interested in running an instance/updating the software/either of those two, it will keep existing. People used to, and still do, post pics on their website. What’s the point of that? People who have friends on Instagram will stay on Instagram. People who have friends on Pixelfed/Fedi as a whole, will probably keep using Fedi. It’s not an all or nothing game.
It's called the Network Effect. Most people, like you, want to be on the platform that everyone else is using. Others want to be on the "best" platform (whatever that means for them) and when they find one they like, they'll start advertising it to others. Eventually, enough people will move to a new platform that it starts making sense for people like you to move there as well.
Remember how everyone used to be on ICQ, then MSN Messenger, etc? It used to happen a lot with messaging and social media platforms until Facebook and Twitter got big enough to start buying and shutting down the competition. It's happening now with people leaving Reddit for Fediverse platforms like Kbin and Lemmy.
There is nothing wrong with waiting for the Network Effect to push you to a new platform, if it ever does. The point of social media is being social - if you're there to interact with friends, you obviously want to be where your friends are.
Getting a teeny bit slammed for your comment, but I think the simpler answer is: you probably wouldn't. If instagram is working for you and yours, then I'd imagine you'll stay there. That's totally fine. This is a thread about federated alternatives, though, so the overall subject may not apply to you.
I like pixelfed because a) it doesn't have ads for 2/3 of the content, b) it doesn't have reels (which IMO go against the entire point of instagram to begin with), and c) I'm specifically not looking for pictures of my friends. For me, instagram is a platform for discovering art from other people. It still works for that, but there's so much other stuff on there getting in the way. Pixelfed is "back to basics" for what I'm looking for in an art sharing platform.