The more of them that remove themselves from the race, the easier the path to the nomination for whoever wins. Don't discount the importance of avoiding a lot of attacks from your own fellow party members. Or the benefit of having someone like Trump attract so much of the attention-- he becomes an easy target for criticism and helps to keep the other candidates from being pummeled by detractors, then he winds up either ducking out before the primaries or being crushed pretty early. He's a highly-effective smokescreen, and for all I know he may be pursuing that angle deliberately.
"Well, evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape- like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered."
-Stephen Jay Gould
-Stephen Jay Gould