Imagine you are a JS dev and github comes out with a new search feature that's really good. it lets you use natural language to find open source projects really easily. So whenever you have a new project you check to see if something similar exists. And instead of starting from scratch you start from that and tweak it to fit what you want to do.

If you were the type of person who makes tiny toy apps, or you worked on lots of small already been done stuff, you'd love doing this. It would speed you up so much.

But if you worked on a big application with millions of users that had evolved into it's own snowflake through time and use, you'd get very little from it.

I think I probably could benefit from looking at existing open source solutions and modifying them a lot of the time, and I kinda started out doing that at first. But eventually you realize that even though starting with something can save you time, it can also cost you a ton of time so it's frequently a wash or a net negative.