Two main use cases:

1. Sometimes you need to charge using a cell phone charger. It'll be slow, but that's fine.
2. Even if you have a 100W charger, the battery will last longer if you normally charge it at only 50 W. Being able to restrict it like that would be very helpful!

Edit: Specific proposal:

Similar to the --charge-limit flag, which allows a user to set to the maximum percentage charge, there should be a new flag called --charge-rate which allows you to enter a max number of watts to charge at.