There are two main reasons why you want your clan to become official.
The first one is your own board where you can make as many threads in as you want. You can make stickys and annoucements, you can close, delete and merge your threads as you may want to and you can choose up to three people to get those power. Clan boads are fixed to a position so they can't get to any second page if you don't post in it for 3 hours ;)
You can even get a privat sub board in your clan board with a password.
You still have to follow the general forum rules though and you have to actually use your clan board.
The other reason would be the Toriclan page for your clan
Toriclans
All official clans get a toriclan page. Toriclans enable you to have a clan tag in-game and displays your clan name below your post count. It has clan war feature where you can fight other clans and it records all the scores for you. It also ranks you against the other official clans.