Hi! Thanks for taking the time to give me some feed-back, or to even send me some money! (Or something else nice.)
Most fields are optional, unless you are a business actually registering a piece of software to be able to continue legally using the software, in which case I obviously need the minimum details required for me to send you confirmation.
(And if you do fill in all the details, rest assured that I'm the only person who would ever get to see the information, and if I ever do send you an unsolicited e-mail, it will be explicitly to do with the software you have registered.)