I have a couple "redundant" bikes. If I was pushed for a reason for why they're worth keeping, it would be a "best tool for the job" sort of idea. For example I have 3 "primary" road bikes. One is a great climber which I usually use for races, and more recently is dedicated for Zwift. One is an aero road bike and more comfortable, so I use it for longer rides or certain flatter races. And one is sort of a beater that I use for any foul or potentially foul conditions, oddball solo rides etc. A big thing I find is that cleaning and maintenance events between the three end up much further apart because the wear and tear is being spread across all 3 bikes. It's the same with my 2 main MTBs but they are much more situation specific: an FS geared 29er and an SS rigid 29er. Going to waxed chains on everything has helped a lot, especially MTB. No need for deep-cleaning drivetrain after almost every ride.
I don't think having multiple bikes drives me to ride more, though it might help. At least when they're clean. There is something about looking at a bunch of dialed-in bikes waiting to be ridden that inspires me to jump on. When everything is grimy and in need of attention it might have the opposite effect
