It's necessary because we have elections with geographically based electorates, so your address determines which ballot you should receive (and so they know how to fairly resize the electorates). So you need to tell them somehow where you live. Also, the roll of voters has to be printed out many times to use at "check-in" on voting day, so you need to tell them in advance of this printing. They need consistent rolls because after the election they reconcile the various copies after the election to question/fine people who either didn't vote or voted more than once.

For people not on the roll, there is a special secondary method of voting called a declaration vote that allows you e.g. to vote even if you haven't enrolled. But this process wouldn't scale very well - these votes aren't counted for days after as they need to check the details you declared - and is intended as a safety net.

This seems a bit like Chesterton's fence...