I'll add my two cents to an already fine list of reasons:
1. I hunt, and different guns lend themselves to different applications. For hunting alone, I like and use a .22 rimfire, a .22 centerfire, a .30 caliber carbine for woods carry, a .30 caliber rifle for beanfield shots, a 20 gauge for birds, and a 12 gauge for furry small game and turkey. Throw in a .44 mag pistol for the challenge.
2. I carry, and again, different guns lend themselves to different situations. .32 mousegun for "all the time," 9mm compact for deep cover carry (ankle, deep cover shoulder rig, belly band), .40 S&W mid or full size for belt (inside or outside). Rule is, "Have a gun," and different contexts may dictate different form factors.
3. If you like to shoot targets at long ranges (>500 yards), you'll want a gun or two specialized to that application. You could hunt with a long-range rig, but I'd probably mount different optics for the different applications. And a heavy barreled rifle isn't something I want to schlep through the deer woods.
4. I have kids who like to shoot, and I like to introduce others to the sport. So that's not one .22 rimfire; that's several. Same with the shotguns.
5. Guns break. Had a Glock develop a malfunction during a training course (bad extractor); good thing I had a Smith&Wesson along just in case. On a hunting trip any distance from home, a couple of rifles or shotguns go along for the ride.
Those are all practical reasons for multiple firearms. Add to that, "I like them," and you'll quickly find a desire (if not "need") for more. However, I only collect "shooters." If I don't have an application for it and it doesn't go to the range with me a couple times a year, it's likely to get sold or traded.
