STEEM is a fee-less platform. Here is bit more info on it. It requires 5 STEEM to be burnt to create a new account/wallet. To promote the platform, Steemit.Inc is doing this for free and they have to make sure that nobody is going to game the system. If one person make 100 accounts, that's 500 STEEM out of the pockets of @steemit
That's why they collect info.
Btw how much steem are there in total supply . I heard that sbd's are infinite . Is steem infinite too ?
SBD is capped at 10% of the marketcap of STEEM AFAIK. SBD is a debt instrument that is a claim on 1USD worth STEEM. The debt level is meant to be at 10% of the total value of STEEM. Hence 10% of the marketcap of STEEM. In about 10 years STEEM inflation will go down to 0.95% and it'll stay there. So both are infinite (sort of).