NEX 7 benefits of primes over zoom

All,

Even though I've had an SLR since the 80's I consider myself a beginner in many ways especially when I read the various posts on this and other forums.

I just purchased a NEX 7, Tamron 18-200 lens and a Sony 30mm macro lense. I'm enjoying the camera vary much.

Could someone explain something very basic to me? A lot of folks discuss the Sony 50mm 18 E mount lens and describe how great their photos are.

As s rather unsophisticated user (i do have to admit this), how would I better understand the benefits of using the sony 50 mm lens over my tamron at 50mm?

Again, I realize this is basic and maybe shouldn't be on the NEX forum but maybe it is because I am beginning to better undersand some of the nuances that arise with a better camera that I am asking this question.

Thanks in advance to anyone who is willing to answer this question.
Rick