Separate Forum/Sign-up from BT Site

Why is it that some sites have a separate (from main BT site) forum that has an additional sign-up page that one has to register on, as well, to post. I have never asked anyone but am curious as to why this is. Does it save some kind of cost, money wise? Keep load down on main site? Seems to be prevalent on Asian sites and then here and there on others. When I see this, it annoys me and I rarely sign-up a second time. I have seen this scenario in a couple different ways. 1) it appears to be integrated with the site and 2) Opens a new window and appears to be totally separate from site (domain or whatever (forgive my ignorance)).

The simple question would be what is the benefit of having the two. I wouldn't think it would benefit the user and it doesn't benefit the site for users like myself who just say fookit and simply leech.

To keep the forums alive even if the main site goes down.

why some sites log you out when your session is finished? why a lot of sites require to logon again at some point even if your ip didn't changed?
why using a captcha during the first logon attempt?

..i have an answer to OP's and mine questions. cause admins are idiots.

ps it's not a problem to have separate forum from tracker, it's obvious that vbulletin is better than regular tracker forums engines (except maybe torrentpier that was made a forum-like), but tracker<->forum registration and navigation should be absolute seamless

Because its a right pain in the arse to code it, they are not compatible with each other, were never designed to be together, so you have to hack it to work, or bridge it anyway, so its not ideal and its a hell of a lot of coding to get it just like ours, sign into tracker you are signed into forum auto, and you should never sign into the forum as that will log you out of the tracker.

We did take the login off the forum, but every little change we made, would just put it back again, so it was pointless.

and the reason we done it is because the forums on trackers are naff (no offence to whoever coded them, but they are) i hate them.