Cropping thumbnails so it looks better

I am trying to sort out the crop for a thumbnail.

I go into media library.
Select the image.
Click edit image.
Select "thumbnail" under apply these changes to.
I draw a 150x150 square over the part of the image I want the thumbnail.
I click crop.
I click save.

This doesn't do anything. The image afterwards is exactly the same. Am I misunderstanding how this feature works or is somehting else at play here?