types give you option, therefore enhances predictability (type hint)

concurrency and async / await and parallelism

If you are using third party libraries that tell you to call them with await, like:

results = await some_library()

Then, declare your path operation functions with async def like:

@app.get('/')
async def read_results():
    results = await some_library()
    return results

You can only use await inside of functions created with async def.

If you are using a third party library that communicates with something (a database, an API, the file system, etc.) and doesn't have support for using await, (this is currently the case for most database libraries), then declare your path operation functions as normally, with just def, like:

@app.get('/')
def results():
    results = some_library()
    return results

If your application (somehow) doesn't have to communicate with anything else and wait for it to respond, use async def, even if you don't need to use await inside.

If you just don't know, use normal def.

Note: You can mix def and async def in your path operation functions as much as you need and define each one using the best option for you. FastAPI will do the right thing with them.