First of all, a right and a need are different things. I don't need to tell you why I need another gun to justify buying one. My worst run-in with the law was a speeding ticket and that was fourteen years ago. There's no reason why I wouldn't be allowed to have another gun.
That said, if you know what the hell you're talking about, you tend to understand why people need several models of the same basic instrument for different purposes.
People may ask me why I need so many different cameras. Well, I have a few 35mm cameras, a 645 camera, a 6x6 camera, and a 4x5 large format camera. Why do I need all that? Well, in most cases, I don't. I have one 35mm rangefinder because it's light, the lens is sharp, I can focus it particularly quickly, and the film is cheap. I also have two 35mm SLRs because rangefinders suck at close focus. I have my 6x6 because it produces gorgeous images with minimal grain even at high speeds. I have my 645 because it's a happy medium between 35mm and 6x6. I have my 4x5 for when I want hyper resolution or selective focus or any of that other shit.
Likewise, I have my Winchester rifle and my .45 Long Colt because I'm into Westerns and I have and will use them in Western productions. My rifle is perfect for deer hunting if I ever decide to do that. My Colt is nothing to trifle with if I need to use it against a home invader or something of the sort.
Still, the snub nose revolver I have coming serves a different purpose. I can't conceal carry the two guns I have right now. I can carry the new gun anywhere I go.
I'm also not dismissing the possibility of getting a shotgun for home defense. Those guns are different functionally from anything that I have at the moment.
In general, when you learn about anything, you realize that there are functional and nuanced differences between the tools.