What is the point of the python application config "protocol"?

[bunny-therapist]

Docs say

protocol String; hints Unit that the app uses a certain interface. Can be asgi or wsgi.

But it is not a required field, just a "hint", and things seem to work for my asgi app even without it. What is the point of specifying this hint? Does it speed up start up? Does it avoid errors? Is it ever needed or does unit figure out the protocol on its own?

Edit: I found this:

Choose either one according to your needs; Unit tries to infer your choice automatically. If this inference fails, use the protocol option to set the interface explicitly.

But I do not quite understand how often this fails or what the inference logic has been tested with. If it fails, what kind of error would one get?