SCRIPT FORMATTING WORD PROCESSING SYSTEMSLike their title, they're expensive ways of doing what screenwriters do for free. There are a number of systems out there which do the formatting for you as you go along. But sometimes they are rigid devices which won't bend to your needs. And some are sloppy and don't know how to gracefully cut a page cleanly. And some don't have the ease and comfort of the major word processors like WordPerfect and Microsoft Word.
A screenwriter who knows how to effect the feeling of a reader might want to cheat the system to create the effect. Sometimes you don't want the car crash to be on the top of the next page and sometimes it's great to have the crash come after you turn the page. Most format systems don't care about things like that. A good screenwriter does.
Then again, they sell a lot of those Script Formatting Word Processing Systems. Try both way. See which serves and suits you best.