Both to prevent silly games and for similar reasons as the "no brown M&Ms" rider in Van Halen's venue contracts: if you read and followed the instructions in detail for small things like formatting, you read and followed the instructions for the things that actually matter as well.
Consistency mainly. Judges want to be able to easily understand what they're looking at and for it to look professional. Double-spacing gives room to write in changes or notes. Line numbers let you reference stuff easily.
