The banishment spell allows me to send a creature to another harmless plane of existence, where it is incapacitated.
Assuming that all of the creature's equipment travel with it, what other non-equipped objects would travel with it? For example, a creature bound by rope: does the rope go with it or would it be freed when the spell ends?
The rules do not attempt to address every possible situation, and this is one that they don't. The DM should make a ruling, and that's how it works.