Is high back swing an essential element to achieve good bowling? How does a bowler create a good rev?

No, a high back swing is not essential to achieve good bowling. It can actually cause a lot of mechanical issues. Most bowlers with a really high back swing pull the ball up there which is counter productive to good bowling.

Good revs is generated by a strong hand position and a slightly bent wrist and then unhinging the wrist at release as the thumb comes out of the thumbhole.

Everything inferno said is basically spot on. There is one purpose for a high backswing, and one purpose only; ball speed. High rev guys like PBA stars like tommy jones and jason couch, have high backswings to compensate for their high rev rate. You don't have bowlers who are exceptional when they throw the ball slow, but have a lot of revs, and vice verca, you don't have great bowlers who throw the ball really hard, but with little revs. Obviously there is some leeway in there, such as slowing a ball down for it to hook more and speeding it up to hook less, but that's the main principle. And most importantly, it is not essential to be a good bowler. Norm duke is living proof.