If your instance is badly run, or if it is run in ways that don't suit you, your experience will suffer.

Instances have human beings actively moderating them and deciding what is acceptable behaviour. (Instances that aren't moderated rapidly turn into toxic spam factories.)

There are also questions of languages, technical reliability, additional features like larger character limits etc.

Additionally, there's the local timeline where you can meet other like-minded people.

It's what @FediTips said, but also, I see it as a kind of "rather have and not need it than not have it and be in a situation where i do need it" feature.

The local timeline shows posts from only your instance while the federated timeline shows posts from your community and everyone else's, so in general, you're more likely to see posts from your own instance than outside instances.

So where do we find the people outside of our instance that we have chosen to follow?

If you follow an account, its posts will show up in your own timeline, even if the account is on another server.

If you search for stuff, it will show search results from across many servers.

EDIT: This guide might help explain things https://fedi.tips/what-are-the-local-federated-home-timelines-how-do-i-stop-them-scrolling-too-quickly/