I know I never know why people would hack forums? I mean what is there to gain?

A lot of script kiddies (Because those are the people who will go after your site, unless they have some issue with your community), will hack the site for the IP addresses, emails, and other contact information (From the database).

The niche my main site is in, about all my competitors were hit by the same hacking group, as the guy who ran it was stealing all their financial information by social engineering support for their email provider and/or figuring out their secret questions.

that kid from 4chan wasn't caught by hackers. there were clues in the video that allowed people to figure out who he was. he wasn't a hacker either. its not hard to use proxys and make your IP very hard to track.

I was hacked recently and the guys continues to email me demanding I pay for this and that, that I send him $100USD and just continues to demand payment from things otherwise he will hack and delete everything again - I don't know what I can do about it.

well contact the police as its basically blackmail.
Dont ever give them money - just walk away.You should always have a backup so you can start afresh on another host.

oh God!
It would be nice if you could knock on their door as they're posting stuff online and say "are you [username] on my forum?" .... and when he says yeah ... punch him as hard as you can! :D