So say you have your own personal instance, and you use that to follow community news on lemmy.world. If throughout the day that community receives 10 new topics, 50 comments and 100 upvotes, it would have to make 160 calls to your server.
So when you decide to read those 10 topics (if you even read all of them), you would then make roughly 10 api calls.
You would be saving those last mentioned 10 calls by using your own instance, but at the cost of 160 calls made throughout the day.