The main reason for a Premise is to keep your work true to the plot, and to keep you from wandering off in several different directions in your storyline. I'II explain by giving the definition of a premise:
The Premise can be compared to a Theorem: A proposition embodying something to be proven. Simply put - whether it's a novel, short story, or a screenplay, all works implant a suggestion in the reader's mind. And these suggestions come from ideas the writer is willing to prove.
Going back to the past, crime does not pay was a popular theorem in movies considering the amount of gangster movies made during the thirties and forties. Gangsters always paid in the end with a stiff prison sentence, so the writer proved his point without beating people over the head with it. However, today you'll see a lot of movies in which the writer believes that crime does pay and writes a screenplay to prove his point and to convince the viewing public that he/she is right. This is the result of changing times.
In the 30's and 40's we had censors who ruled what went onscreen or in print according to a code of morality and ethics. If someone wrote a work showing crime paid, it either was never made into a movie or never printed as a novel. But today that code doesn't seem to exist. So the sky's the limit in premises.
But whatever your premise--good always wins over evil, young love is ill fated as in Romeo and Juliet, or our cruelty towards the American Indian caused us many problems, as in Dances With Wolves, be sure to prove it silently. And choosing a premise is extremely important because it gives you direction. Once you realize the theme of your work, you'll do a better job of sticking to it and not wandering all over with your plot.
For instance: in Unholy Alliance, my murder mystery, the premise I chose was sharp investigative skills and attention to detail when it comes to forensics will help catch the perpetrator every time. If you don't believe I proved my point, pick up a copy and see for yourself. My premise kept me on the straight and narrow. Works everytime.
