as salaamu alaikum,
yes, as otheres mentioned, it helps keep out the spammers who use automatic programs that can register and post spam on forums for them.
although even this doesnt keep out all spammers. some are determined enough that they actually go through the trouble of registering, just to be able to post a couple of spam threads before they inevitably get banned.
so yeah... its an unfortunate necessity. but you typically only need to do it once, unless you are reating multiple alter-egos. in which case, one might consider that a form of spamming as well

meraj