What is the function of presser bar on sewing machine?

It makes the needle move.

Actually the pressure foot works in conjunction with the feed dog mechanism, as well as in synchronicity with the needle movement, and the looping hook. In a zigzag for instance the side to side movement creates a taller or shorter 'Z' based on distance the feed dog moves the material. The feed dog is simply a jagged stepped piece of steel which travels one direction in an elevated motion, 'pushing' the material through the machine. The feed dog returns it's travel in a lowered position, thereby allowing the material to pass in one continuous direction. Often it is the pressure foot which is the final adjustment in preparing ones machine, and is adjusted with more pressure for materials with a more stable nature. It has been my experience that if a fabric is not smooth ( IE. a pocket on a pair of jeans), that loosening the pressure foot allows for easier control whilst sewing around bumpy edges. In fact, I must admit I am more of a technician than a sewer ( what do you call one who sews?), but I've found that you can increase the number of stitches in a given area if you lessen the foot pressure and pull the material back slightly- then let the material go back across the same point again. This of course is the bane of every sewing person ( smells, oops spells better than sewer). If one could only sew a cloth with the same accuracy that their machine was prepared to handle there would be far more people still making their own cloths, or at least fixing them. The bottom line is that the less you sew the more needles you'll break. Practice practice practice.