I recently bought my second Reflex. And as for the first one, the 1st "option" I bought for it was a protect filter.
The cap on the lens is great, but it is not attached to the camera or the lens. Therefore, it's easy to loose. More over, that's always the bit you forget to remove before shooting, and then your photo opportunity is gone.
So this protect filter is a kind of transparent cap for the lens. It takes the shock, the marks, the dirt, the finger print, and your lens stay clean.
From time to time, I put the protect filter in the dish washer to clean it, and if it breaks, it's only few £ to replace. Much less than replacing the lens itself.
Definitely a must have.