This is actually a very good question. I have multiple guns for many of the reasons stated already.
My first gun was a Winchester Ranger 20g autoloader. I bought it in college because I took up hunting. Only thing I ever shot with it were clay birds, but it was a good gun none the less. I ended up selling it after a few years of not using it and don't regret it beause I've never had a use for it it since.
My second gun was a Savage model 10 chambered in .243 Win. I used this gun to take quite a few ground hogs and it was a lot of fun at the range. I sold this one too after years of not shooting it and I needed the money. I do regret selling this one because it did have the memories of fun times with friends.
My third was a Mossburg 500a 12g. I bought this when I was around 19-20 and turned it into a defensive gun after a scumbag ex of one of my female friends was threating me. He was a woman beater and a druggy so the threat in my mind was very real. That threat went away after he ate his own bullet a few months later. I still have that gun and it is still configed for HD.
My fourth was a Ruger P-95 DC. I bought this for home protection and it was my first handgun. I had nothing but problems with it and since Ohio was looking at passing it's CCW law about a year and a half ago, I bought a Glock 27 for both carry and for HD since the Ruger had reliability issues and I couldn't trust my life on it.
I sold the Ruger off to help pay for my GF's Bersa .380 which she decided she liked. It was her first pistol and she didn't have much money to spend. She still has it and it's a great shooter. I don't miss the Ruger one bit either.
Then I moved to Oklahoma and shot my first USPSA match. Now had I not shot in USPSA, I probably would have been satisfied with just having the G-27. It fits the home defense/carry/range bill just fine. I also have to 12g for HD so those bases are covered. However, after shooting the USPSA match, I decided I wanted to get a gun dedicated for that. I was tired of having to strip my defensive ammo out and then having to rechamber them after every match. So I bought a Glock 35. I personally really like Glocks. They are for ME what really works and feels good to ME. I went with the 35 because it shares ammo with the 27 and I can use the 35 mags in the 27 if I so choose to.
The same day I bought a used Glock 17 the store had in it's case mostly because it was a very good deal and I couldn't pass it up. It was a mint second gen with a rediculously low round count through it. I had been wanting a G-17 because 9mm is so cheap but wasn't planning on getting one at that time. The G-17 doesn't really fit a "useful" niche for my needs. I already had my carry/home defense/competition needs covered so this gun is more of a fun gun.
A month later I bought an AR-15. I've always wanted one and since I didn't have a rifle of any kind at the time, I got one. I can play three gun now so it does have a practicle use for me. They are darn fu to shoot as well
My last purchase, I bought a Rock Island Armory 1911 Government model.
As much as I like my Glocks, they do lack somewhat in the asthetics department. They aren't ugly, but just plain and utilitarian. Which is fine but I wanted one gun that I could dress up and the 1911 is a handsome gun in it's basic lines and can be made into a visual work of art.
Well, I thought it would be a good idea to have a gun in a few various calibers, so I bought a Colt Officers for .45, a Browning BDA for .380, Ruger Redhawk for .44 mag and so on and so on. Well then, I decided it would be a good idea to own guns in various action types, DA/SA, DAO etc. etc. Then, of course, I had to begin my search for the ultimate carry gun. Anyway you get the picture, one thing just seems to naturally lead to the other until you look at all of these guns that you (at least me) don't really have the time to shoot but you're still very glad that you bought them if for nothing else but admiring and fondling them. 
