In most cases, they add nothing to the sentence, and should be eliminated. Jerry raced over to the apple tree and began to climb to the top. Jerry raced over to the apple tree and climbed to the top. When the car was dry, Dad proceeded to wax it with a chamois. When the car was dry, Dad waxed it with a chamois.
Normal speech is full of stops, starts, pauses, and the inevitable ers, hums, ehs, and buts.
If a writer were to write exactly as most people speak, he would end up with pages and pages of dialogue so boring no one would want to read them. Instead, the writer must use realism , a tool which creates the illusion of reality. A good writer can give the reader a sense of his character's personality without resorting to the boring repetitions found in real speech. I hollered, settling down into my favorite chair. The ball game just started.