I don't think there's an explanation of who the pirates are and whatnot.

Different items have different likelihoods of spawning certain pirates (except Ambling Pirates, you'll get tired of seeing him) and most of them only start spawning after a certain floor, so if you're having trouble finding a specific one, try looking at those - this guide[gamefaqs.gamespot.com] covers that (it's in the MISC. > EX section, though you can just use Ctrl+F and search for "Treasure Map"). Keep in mind that you'll still be at RNG's mercy regardless, so it'll still take a while for you to encounter them. That's not even mentioning that you have to beat/steal the map from them, since the ones that spawn at higher floors will have very high levels (you can try going in a low rank item to help with that).
edit: I forgot to mention, and you might've already realized that, but pirates only spawn if you wait at least 1 turn in the item world. The chances seem to decrease after every following turn, so just wait ~2-3 turns and move on if none spawn.

As you've read, if you collect all (16) Treasure Maps, you unlock the Land of Carnage (LoC), which is basically another hub (like Holt Village), but the enemies in stages/item world there are much tougher (it's a flat multiplier to their lvs and stats, though I forgot by how much) and IIRC you get twice the EXP there. There are a few more nuances to it (for example, you can only steal rank 40 items from the Item God if you're in the LoC, some superbosses can only be fought in there etc), but that's the gist of it.