I put a new battery in my truck last night and I did not put the battery cover back on. It does not fit tight and I like the look of it with it off better. What is the cover for?

I don't know why it would be necessary. It may be there to trap what gasses leak out of the battery, keep them from rising to the paint under the hood. Me, I like the looks of the cover. I don't see where leaving it off would hurt.

You are absolutely correct! The covers are for trapping escaping corosive gases that might leak out! When I installed my new Optima Red Top last year, I left the battery cover sitting on a shelf in my closet. I'm not worried about the Fluid less Optima Leaking out anything! Plus I like the look of the Optima with the cover off.......

its also to keep the battery somewhat warm for places like alaska and where it gets real cold. mainly for starting in cold weather.