This is mostly because NASA wants there to be an escape seat for every astronaut in case of an emergency that forces the evacuation of the station (or say, a me

This is mostly because NASA wants there to be an escape seat for every astronaut in case of an emergency that forces the evacuation of the station (or say, a medical emergency). Even if they want to relocate a Dragon to another port, everyone who rode that capsule has to get back in for the relocation, so they always have a way back down. During the space shuttle era, initially this wasn't a concern as the plan was to have the crew stay in one module until a rescue vehicle could arrive, but after Challenger, there was an extra Soyuz for that purpose.