The same reason why most retailers ask for your address even if its for an electric purchase. I believe it used to help fight against credit card fraud.
Also... what you're buying is akin to a subscription, i.e. Apple Developer membership/agreement for 1 year. It's treated differently than buying candy at the supermarket. There is likely a legal requirement to establish identity of the "subscriber", and thus Apple may be collecting the legal name & registered address. Moreover, if you misuse the subscription and say upload confidential pre-release information to PirateBay, they need to know where to send the Feds