The feedback that is received by your registering and re-registering gives avast a good idea of the number of active avast users there are and that helps them ensure that there are sufficient servers to do VPS and Program updates. This way you don't suffer the update problems connecting to update servers associated with a number of other AVs and programs.
Recently avast clocked up its 20 millionth download of avast 4 home (offer of 20% discount on Pro version for a limited period
http://www.avast.com/eng/promotion-discount-offer.html), now they aren't all active so the registration process helps with planing.
As Alan said a small price to pay 1 minute (tops) of your time.
