So then there is a purpose to the legal requirement, it documents the submission date.
Perhaps in theory it does. But it really doesn't.
1. The statute doesn't specify that the date to be filled out on the envelope is the date the envelope it is sealed. Apparently, any date will do.
2. There's no way in hell to verify that the date written on the envelope is the date it was actually written.
3. The verification of the submission date is one or both of these dates: (A) the postmark; and/or (B) the date it is received at the election office. If I receive a letter in today's mail, it's ludicrous for anyone to suggest that it's remotely possible it was mailed next week.