Perhaps the single most important thing to know about screenwriting is that the actual product that screenwriters are selling is emotion. In order to give your audience their money’s worth, you need to make them experience as wide a range of emotions as possible. The way to do this is to build up tension, then release it, and then build it up again. The longer you can keep your audience on that kind of emotional roller coaster, the more satisfying their experience will be.
One script that I read started the protagonist down a dark, difficult path right at the beginning and then it only got worse and worse from there. The screenwriter never brought the protagonist back up from that low level, so the audience would have only experienced the emotion that took the protagonist from his beginning level at point A down to his concluding level at point B. If the writer had relieved some of that tension, then the audience would have experienced the full emotional range from point A down to point B and then back up to A, and down to B again, as many times as he liked. That’s a much greater emotional distance to travel than just a one-way trip.