This is a real thing? I just use a paper tissue and throw it away...
I guess the thing you're really surprised about is the number of men using the same rag multiple times and treating it like some kind of trophy. Clearly it is a thing, as plenty of men have posted about it,
but that doesn't mean it is representative of the male population in general; this thread has "cum rag" in the subject - the people who answer it are self-selecting. So you could call it some kind of fetish and there is nothing wrong with that.
As for how that comes about I can think of at least a couple of possibilities.
Firstly, when I grew up, as a teenager I didn't really want to advertise to my parents that I was masturbating, hence using pieces of paper that could go into the bin rather than cumming on things that would need to be washed. But someone who did use something made of cloth may then feel like they need to hide it to avoid mum noticing the cum when she came to wash it. Thus begins the behaviour that turns into a habit that persists into adulthood.
Then, maybe there is an element of rebellion about it. As toddlers, we get toilet trained. We learn to perform those bodily functions in private and make sure whatever comes out is safely disposed of in the toilet so that this nasty stuff doesn't make our parents nice clean house dirty or smelly. Then, come puberty, we have this new bodily function, one which is much more fun than the other too, but which also involves something coming out of the body. It is tempting to treat this much the same as the others, hence the guys who only masturbate in the shower so they can quickly wash the cum away.
But, at some point, we may start thinking differently. We start to realise that, despite sex and masturbation not being done in public, and despite cum being ejaculated from the same penis that piss comes from, cum is something very different. It isn't just the byproduct of our body's metabolism; it is something that gives us children, if we want, and is actually a kind of essence of us. Most guys who have expressed an opinion like seeing it spurt out at the end of masturbating. It is something that feels sexy and a marker for our virility. So, in that context, a trophy cum rag perhaps is not so strange.