[Hollywood]

It's all about money, first for the studio and then for for filmaker. Most films are cut so that their runtime allows as many showings as possible in one day (theater). If a movie is 3 hours long, you can't show it as much which means less box office take.

Then, to appeal to dvd owners, the filmakers add footage and call it a director's cut so you buy the DVD. In some cases they release multiple versions and some geeks buy both versions.

Also, sometimes scenes are cut because they don't work, or they slow the movie down, or the acting sucked.

Some movies to benefit from a DVD release are Aliens and Abyss, both by James Cameron. The extended versions are much better than the originals. I actually had the original laserdisc director's cut of Aliens, way before DVD's were even thought of.