It's actually a good thing in my view. Republicans are desperately trying to stop this effort because lower registration of youth yields better outcomes for Republicans.

They ask because it's a matter of convenience. State Department of Motor Vehicles routinely handle Voter Registration requests. So when you're there they ask on the forms, "Would you like to register to vote?" And if so, they ask if you want to choose a party or remain independent. Many states including Pennsylvania have closed primaries, which means if you are officially registered as as a Republican you cannot vote in any other party's primary.