Lajos Egri quotes:

No two dramatists think or write alike. Ten thousand playwrights can take the same premise, as they have done since Shakespeare, and not one play will resemble the other except in the premise. Your knowledge, your understanding of human nature, and your imagination will take care of that.

Everything has a purpose or premise. Every second of our life has its own premise, whether or not we are conscious of it at the time. That premise may be as simple as breathing or as complex as a vital emotional decision, but it is always there.

No, you don't have to start your play with a premise. You can start with a character or an incident, or even a simple thought. This thought or incident grows, and the story slowly unfolds itself. You have time to find your premise in the mass of your material later. The important thing is to find it.