As others have said, because I can. For years when I was scruffling to make money playing, I had three guitars, my F-212, because it really is part of me, my Mark V, because playing classical guitar entertained me, and my gigging guitar. The latter changed regularly depending on the gigs, I bet I went through at least 15 guitars between '72 and '80. I wish I could have kept them all, some of them would be worth a pretty penny today, but they were just tools of the trade.
They all sound different. Over the last few weeks, I've been rotating between the six strings, a different one each day for a while. I think the F-50 is the most versatile, the D-50 best for bluegrass and flatpicking, not sure where the F-40 fits. I also have a LoPrinzi LM-20 (Martin D-35 clone) that is okay for flatpicking and a Harmony Sovereign that is a nice blues guitar. The Masterbilt is very mellow, but I keep meaning to put on a metal nut and set it up for lap steel style playing. Even though my old F-212 is probably my most comfortable to play, it needs a little work so I've been using the 512 for all my 12-string stuff.
I also have two acoustic archtops, an Artist Award and a 1930s Metro B, different as night and day. The Metro has a sound a lot like the old Epiphone that David Rawlings plays, the AA is much more modern sounding, if that makes sense.
Brad
(Guilds that is)
