I can't enlighten you on why other people use screenwriting software, but I can speak for myself. A few hundred dollars to make the formatting job a little easier is worth it for me. Making changes, adding or omitting scenes, creating a production breakdown and generating new production pages is much easier for me when using software.

There are several things that screenwriting software can do that you can't do using the tabs and settings in a word processing program. For example: You can type the first letter or two of a characters name and it immediately is placed in the correct place on the page. In caps. You can do the same with slug lines - very convenient when you are writing a script that uses the same location several times. The right dialogue margin is always correct. I like that screenwriting software always breaks the page according to my preference, never having the characters name on one page and the dialogue on the next, or the slug line on one page and the action on the next - very convenient when rewriting. Not essential - as you point out, you can do some of this for free with a little time - but I like the time saving. Then there is the production mode in Movie Magic. Essential if you are a small operation - producing, directing and even doing the budget yourself. With a couple of click of the mouse you can instantly see how many scenes any given character appears in and how many lines they have. You can see how many INT scenes and how many EXT. You can see how many total pages take place INT. BOB'S APARTMENT which really helps in scheduling. You can "tag" the script to create production reports on what props you need and in what scenes. Again, you can do all that for free with a little time and not spend the couple of hundred bucks for software. For me, I'm willing to pay for the time saving and connivence.