It's like ELO. Really doesn't mean much but you obtain that "arrogancy" of "I'm the Supreme Being of CHF" or something like that. Plus, it looks cool :3
That said, having 1000+ wincode guarantees you to get any mystery rare character in game. And 10% chance to get "Old Blood" (pretty much a vampiric lord hero with steroids on all stats). (and you have access to any mystery character with 50% chance)
Anyways, the most obvious is the extra 25 gold. However, you just need 25 win code for this...
Best hero is in 350 win code slot though. It has the name of the owner and it's really really unbalanced. Fortunately, it seems many veterans don't get that hero as much so because that hero is not balanced (does not die...)