This is a question reminds me of one I hate hearing from liberals: "Why would anyone need more than one gun?" It's like saying, "Why would you need more than one book?" When you hear someone ask why anyone needs more than one gun, you realize you're dealing with prejudice rooted in total ignorance.
I'm also disappointed when I hear someone say they got one gun--a pistol--to protect their home. For carry, sure. But to chase burglars, when you could have a 30-round rifle? No way!
But I digress, since the OP asked something different. The question is interesting to me, because I have been trying to figure out exactly what I need to have in order to do all the things I might reasonably expect to do.
I used to buy what I wanted, not what I needed, so I ended up with items that weren't all that practical. Examples: two Smith & Wesson revolvers, a Colt .38 Super in bright stainless, and a Romak III (PSL). These days I occasionally think about utility.
I bought a .17 HMR because I thought it was a great cheap way to learn how to use a scoped rifle accurately. That worked out really well. It also turned out to be a great squirrel gun, although it's not right for my area. I had a Sweet Sixteen which worked much better.
I wanted to be able to kill squirrels and possums quietly, so I got an air rifle.
I wanted to shoot coyotes without much recoil, so I got a .204 Ruger. I wanted to be able to shoot them (and other varmints) at night, so I got the best night scope I was willing to spring for. I read the hunting regulations and found out I had to use a .22 if I wanted to shoot coons at night, and my existing .22 was not good with a scope, so I got a second .22.
When I shot squirrels, I found that they sometimes hit the ground wounded, and I didn't want to blow their heads off with shotgun shells, so I bought a .22 pistol. I actually needed that. You can stomp on them, but it's a little gross, and they may scamper around and make it difficult, so I like the pistol.
I want to be able to shoot hogs and deer, and people say the .204 is not really a hog gun, so I'm thinking I would like to try 6.5 Swedish, which supposedly works on just about any large game in the US. I have an LR-308, but it seems a overpowered for pigs, and it's heavy. I believe it would be illegal here for deer because of the magazine size, but maybe there's a way around that. Anyway, a little bolt gun with less recoil sounds better.
Then there are my self-defense guns. I wonder if Joy Behar would understand if I told her I needed a semiauto rifle even though I already had my granddad's aluminum .22 revolver. And what if I told her a compact Glock needs to be supplemented with a full-size one for times when I have more room to conceal it? What if I told her I need a 9mm compact because the 10mm is hard on my dress pants?
These days I'm trying to identify niches and fill them. I guess later on I'll go back to buying whatever looks like fun.