Why is high blood pressure needed in the glomerulus?

I believe what you are referring to is what is called Glomerular capillary blood pressure.

This is pressure that is exerted directly by blood and is termed as fluid pressure. It is exerted by the blood within the glomerular capillaries and is dependent on two major things namely:

(1) The contractions of the heart

(2) The resistance to blood flow which is offered by the afferent and efferent arterioles.

The main purpose of this pressure is that it is the major force that produces or effects glomerular filtration also known as ultrafiltration, a process in which both fluids and soluble materials are forced out of the capillaries and into the Bowman's capsule to become urine.