When would I want to delete a command assignment from a control?

When you delete a command assignment from a control, you completely remove the command assignment for that control, leaving the control unassigned to any command. This might be useful, for instance, if you wanted to create an entirely new game profile for a particular game.  

What if I just want to use a different command assignment for the control?

You can change the command assignment for any control from one command to another. See the help topic "Assigning a Command to a Control."