well for example you might want use them to carry some water around.
but if they not empty then how would you get the water in there, it just wouldnt fit.
It's simple physics, if the bottle is full, your not going to get any water in there.
you gotta empty that bottle out so that you can use it to carry your water.

OP is providing a valuable service,  he will do arduous work required to empty a bottle, so that you will ave a perfect empty bottle ready to use however you so desire