Exactly.
My other bikes are there as spares or for different applications, and of course the N+1 thing.
If I need work done on one bike, I have another to ride in the mean time. I have a rigid SS set up for commuting, another rigid SS as my main trail bike, a 1x10 HT I use for new or unfamiliar trails and an old steel 26er for riding around the hood with the kids. I haven't really found a good excuse (or all the $$) to build a FS, but I just got a new gig at my LBS so that may be changing soon.