Whats the advantage of it being sliding as opposed to just one of the drop type?
Well, as others have pointed out, you can cut wider pieces of timber. It also allows you to pull the saw out and start the cut at the corner nearest to you. If you’re cutting, say, an inch off the end of a piece of timber, it means you can check your cut on the RHS of your pencil mark (and you don’t have to draw a line across the whole of the workpiece with a square).
It also allows you to do “rough” trench cuts (some sliding mitre saws have a depth stop to help with this), but you should remember to use a sacrificial piece of timber (straight and square) to effectively bring the fence out towards you, as the curvature of the blade means that the trench won’t be the same depth to the existing fence. (Or simply flip the workpiece round, though this can mean using the saw with your “bad” hand, which some people aren’t keen on.)