It's great for when you can do concurrent I/O tasks. For example running a web backend, web scraping, or many API calls. FastAPI (and Starlette that it's built off of) is an async web framework and in my experience performs well.

Basically, your program will normally halt when doing I/O, and won't proceed until that I/O is done. During that halt, your program is doing nothing (no cpu being used). With asyncio, you can schedule multiple tasks to run, so if one is halted doing I/O, another can run.

Edit: And AFAIK, the GIL does not come into play at all with async. Only when multithreading.