It is usually done to kill of any latent bacteria/germs in the milk so that is doesn't effect the final cheese and people eating it.
As I use milk that is usually freshly milk that morning, and I know the source that it is coming from is kept clean, I usually don't worry about it for our own use.
However if I were to be selling to the public, it is defiantely a process that I would do.
Personally in quality milk, I don't find that pasturization changes the yield or flavour of the cheese. It is when milk is processed at prolong high temps that the yield and quality of the cheese changes. ie: UHT milks.