The SS started as a small paramilitary unit designed to guard Nazi rallies and top Nazi officials. The Waffen SS started as a small bodyguard unit for hitler known as the "Leibstandarte Adolfo Hitler" the 1st SS division. The Waffen SS eventually grew to 38 fully fledged divisions. The SS truly rose to prominence with the Night of the Long Knives when the other paramilitary organization the SA was purged.

The SS was created because it allowed the Nazis to have their own personal army. The Wehrmacht was a bastion of the old Prussian and German. Christian Aristocracy. As such they could never be loyal completely to Hitler. The Waffen SS was completely Nazi however and would be completely loyal to the Nazis and Adolf Hitler. The Waffen SS provided a counter balance to the regular army.

Source:

The third reich at war

The third reich in power

Both are by Richard Evans and are great sources of info about the Nazis.