What is the point of Application layer testing ?

[punkouter2021]

I have a function written in the application layer so I want to test it and test the data it gets back. But it seems the Application layer you mock the data and it is only used to test if certain methods are run.. So should I then start with web layer and create a controller to call that service ?

I am just wondering mostly is the application layer never used to check the results of the services ? According to the current code that is what it appears to do only.

And I am beginner so maybe its a dumb question