Why you should borrow w bitcoin? For the same reason that you borrow fiat, you should take a loan if you have an idea for a productive activity but you dont have the bitcoin at hand.
This is the right way to think you borrow it for the same reason as you would borrow fiat so if you have a plan to make some kind of money with it then you should borrow but dont borrow without a plan or just for a reason because borrowing will always cost money aswell so check how much return you will have before you do it.