Even though readers might not see all the fruits of your "what if" exploration, your manuscript will ultimately reap the benefits. You'll get a stronger feel for the character by looking into all the possible routes a person could take in a given situation. As you reject or entertain ideas based on the personality of your character, you'll learn what makes them unique. The better we know our characters, the stronger they become on the page. In speculative fiction, the same technique can be applied to develop the world the characters inhabit.
"What if" can also save you when you've written yourself into a corner. Go back to the last scene you wrote and start with the first line of that scene, but as you retype it, ask what if s/he reacted differently? What if they were standing on the street instead of sitting at the diner? You'll find your way to the next scene by exploring the possibilities overlooked the the first time through.
During the first rounds of editing, "what if" is very important. What if you take out that character completely? (This will lead to: What does s/he bring to the table that the manuscript can't do without?) What if you take out this scene? What if s/he reacts differently in that scene? Seeing the possibilities not only allows you to develop the story into the unexpected, but also helps you refine the necessary pieces and eliminate the unnecessary ones.
In every stage of writing, from the brainstorming to the first draft to the edits and rewrites, asking "What if...?" will help you create an unforgettable piece of work.