This just occurred to me... Could this site be a way to measure her popularity?
To see how many register, where they are from and what kind of content they like? This will show what her fans like best about her: music (which songs, etc), fashion, variety, etc. Precious data to help determine her future path.
If we take that a step further you could speculate that they're trying to establish is her following is large enough for a proper non-AKB48 solo career. So basically... If you register to that website you want Tomochin to graduate!
j/k