Hello, I opened a savings account at https://securelogin.synchronybank.com/myaccount/loginUserActionGetOnly and now they have sent me a w-9 form? why? Please be clear.

Didn't you give them your ssn when you opened the account?  They need it to report any interest you earn on it.

If you don't send them the W-9 as requested then they must withhold 30% in federal taxes from your interest income. Comply with the request.

thats weird. i did give them my SSN when signing up. My only concern is that i dont want to pay any money or put this in my taxes otherwise earning any interest rate in that account will be useless.

They sent you a W-9 to get your Social Security number that they need to report interest that you earn on the account. Just fill out the W-9 and send it back to them.