A few reasons:
1. I want to start a new lair, so I give it to a different character and have my main who already has max disk and best gathering rate the new lair to work on. This prevents the constant permission problems I've been having where one or the other's plot permissions get randomly reset every week. I've already had to reset 6 plots this month over the course of each week. When the plot is all silos, this gets very tedious. If the one that owns the plot I'm working on, I don't have to worry about setting permissions, it's easier to log on and start working on the actual lair instead of bringing a disk load in and finding I need to reset the whole plot.
2. In addition, said lair might be flying access only, and I only have so many adults ATM so some juggling is necessary between the aerial and ground lairs.
2. Novians. One character has novians that can be used in another lair I own.
3. RP. The character is moving to be closer to family but I don't want to lose the old location and I'm too stubborn to take a 20% resource hit- that's too many hours so I've just been adding subs instead.