Why??.........I guess because we could??
Haven't done the "doppelganger" bike collection thing yet, but I could understand why one would have a spare, identical bike to have as a replacement or back up. It's kinda like the same thing we do with components where we stash up identical parts .....in case we ever wear out the one installed on our bikes:
.....yah sure, that's the ticket!........
Chombi