That reminds me of the signup process for the Electronic Federal Tax Payment System (EFTPS), which is used for things like estimated tax payments in the US. It's not quite as bad since you don't actually need to print & post the form, but they do insist on snail-mailing a code you need to complete the signup process, so registration takes much longer than necessary. All this to set up an account so that you can pay them!

The reasoning is probably the same in both cases: They want to know that you can receive mail at the given address. There are faster ways to confirm that, though, such as uploading an image of a recent utility bill in your name.