There are seals on the slides for when the slide is all the way open or all the way closed. No seal cleans/drys as the slide moves.
It is just a preference. Plenty of people with and without them. They cost money up front. They do keep the top of the slide cleaner (not clean all the time, just cleaner). They can flap in the wind. And they can break costing more money. They do save a couple minutes each time the slide is pulled in compared to getting ladders/hoses out and spraying/sweeping.
All I can say is mine was worth every penny and the OP quote of $800 is too rich.