Here is my warmup for my 5 rep sets.
2x5 Just bar
1x5 around 40%Working Weight
1x3 around 60%Working Weight
1x2 around 80%Working Weight
I also find, as many people said, it gets the muscles ready for the exercise, but there is also another reason. We all know how important form is, well the body learns form through repetition, specifically in the sub 80%working weight area (according to mark rippetoe).
So the purpose of warmup sets like this is to also hammer the form into you, that way when your doing the heavy sets, your not thinking "ok, make sure im rotating elbows enough, make sure my feet are pushing the right way, ok checked back is arched" while trying to handle hundreds of pounds. Doing enough repetitions, your body learns it, so when you are under the bar going for a personal record, all you have to think is "UUUPPPPPPPPP", and your muscle memory takes care of the rest.
Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure.
"When in doubt, just get really, really strong. It tends to cure most problems in training – and life." - Wendler