They aren't necessary at all. They are added aftermarket for those that want them. I have one on my trailer and love it. It allows me to vent out humid air in the shower and not worry about rain coming in. More importantly for me, it allows me to leave the bathroom vent open when my trailer is in storage. I've found that allowing the camper to air out naturally eliminates the mildew smell that you get in campers.
My camper is parked under a pine tree, so we have the needles that work everywhere. When I installed the cover on my new camper, I used a little foam insulation tape under the vent. That provides a tighter surface against the roof and so far has prevented any pine needles from getting underneath the cover. I've never had a problem with them going through the screen... just underneath the cover.