Paintball bottles Covers are necessary for a number of reasons. Firstly and most importantly, they are used to protect the fibre wrap on your expensive paintball bottle. It's cheaper to protect your bottle with a bottle cover than it is to replace the damaged bottle.
Secondly, they usually have a rubber pad at the base which helps you to keep the bottle firmly attached to your shoulder for more accurate shote.
And thirdly, they look cool.