With the pulling stoke that I've developed the bar usually contacts my shins right off the floor. This is why I wear the long socks and actually under them is a thin piece of plastic and foam and the socks seem to put less resistance on the bar than hitting my skin and it doesn't hurt as bad which I find especially important in training.
The knee sleeves I wear for two reasons. They keep my knees warm which just keeps them feeling better and I have longish femurs and when a sit back and pull back the bar contacts my knees on the way up and again the sleeve exterior is slicker than my skin on the bar. I have been working on pushing my knees out so this contact doesn't happen and I can pull back more, but it just comes a little at a time. I could just use baby powder everywhere I suppose to lubricate things, but I don't really want to do that and the knee sleeves just feel good.