Gym - I can't speak for others, but my hobby at one point was trying to collect a sampling of Mosins from each arsenal, each major variant (91, 91/30, M44, M38, etc) and different nations (Tsarist Russia, Soviet, Finn, Polish, Austrian, Czech, Hungarian...). Got pretty far with that goal, but Mosin prices started to creep up (especially for the less-common stuff). But why Mosins at all? Besides the fact that I like firearms of just about every type, I have a lot of historical interest in the the region, from WW1 though the Cold War.
I also have a US collection that spans back to an 1855 Springfield Musket, 1861 Springfield, 1878 Trapdoor, 1903s, 1903A3s, 1917s, m1 Garands and M1 Carbines of different manufacturers and eras (ww1 and Korea). Below is a photo of some of those... Again, why collect all these... they represent rifles that have some intellectual interest, some functional interest, some emotional interest...
Also, for those who might be budget-constrained, but still want to build some kind of "themed" colletion, Mosins do represent the opportunity to build an interesting collection.
Also, I enjoy shooting the Mosin even for its deficiencies. I like the ridiculously long length of the M1891. The Mosin bolt is an odd but fascinating design - definitely not superior to later rifles or even its peers from the 1890s, but interesting nonetheless. I like the rifles' kick (except for the M44, which I noted before). I have found it to be surprisingly accurate (most examples, but definitely not all). It's just a quirky rifle that many find charm in. Many don't. I also greatly enjoy blackpowder shooting with muskets. I like long rifles. Probably some Freudian thing, but let's take the High Road and leave that alone, shall we?
Again, if you don't like Mosins, or are just indifferent, then these might just be big ugly stick to you. Fair enough, can't argue with that. But to some collectors, the represent different aspects of history from a specific region. If you are looking for a shooter, they might not be your cup of tea. If you are into tacticoolery, they are definitely not your cup of tea.
Anyways, here are some of the rifles I have in my "U.S.A" collection. Feel free to question why anyone should collect a bunch of these too, I don't mind...
I have a buddy who was into collecting Krags. Wish I had the bucks for that. Another big, ugly rifle with a number of deficiencies. But they don't seem get the amount of vitriol we see on threads like this.