Grenadier, you ask a reasonable question, and until a relatively short time ago the answer was not clear to me either. Take my response as that of a rank amateur, albeit one who has benefited from a great deal of reading and discussion over the past few months. Most of that benefit is from my friends at TFL.
Why so many guns?
1. They perform different functions. To be frank, when one is ignorant, the differences in any field are not obvious (don't take this as a personal insult - just a fact). To the uninitiated, most cars might seem similar, as well as most boats, fishing lures, insects, guns, etc. Shotguns, rifles and handguns obviously differ. But, within their categories there are many differences as well. Some rifles excel at long range, others for short range work. This discussion can go on and on, so keep reading on TFL and elsewhere. You'll see.
2. Gun control has become a very major factor now. I know people who are actually buying guns for their children. Not to use, as minors, but so they'll have good quality firearms when they grow up. Why? Because the 'clues' are obvious. Many in the anti-self defense movement intend to disarm us. In degrees, perhaps. But their intent is clear. If one believes in the RKBA and is keeping up with the news, how can you have confidence that the firearms you or your descendants may want will be available in the future. You can't have any such confidence. How many guns then? See #1 above. Include magazines, parts, ammunition, etc. in this analysis.
3. For novelty or collection purposes. Perhaps you've kept grandpa's old shotgun for a keepsake. Or, you collect antique single action revolvers. Or maybe you're a Smith & Wesson fan. People do collect firearms for their engineering and aesthetic beauty. Rather like a car collection, but a bit easier to store. They may never even fire many of them.
Folks here can give you more reasons. But, these are the ones that seem clear to me.
Welcome to TFL. Regards from AZ