It is because of issues regarding film length + limited inventory, not always it is the director that edits the film, and the ratings system. In many cases scenes have to be cut to keep running time at a specific length (can't take ownership of theater completely - marketing issues, impacts sales for the theater and no telling your film will generate the revenue if it was 3+ hours when 2 smaller 120 min films can take that space); content - R ratings are believed by Hollywood to kill revenue, so scenes are cut if too risque; and well if you aren't a big name director the studio normally hires an editor that may edit the film to their liking and not the director's. So, yeah, not just a marketing grab by the studio but part of many more factors - and given most releases these days are light on features, some of us appreciate what (in the past pre-DVD) was lost can now be seen. An infamous example are the scenes cut from the ORIGINAL Star Wars trilogy, including a scene in Empire Strikes Back where C3PO goes ganster and rips off a warning sign to a Wompa enclosure. Snowtroopers get shot and are mauled. Or the Return of the Jedi desert Falcon scene, where Luke and Han part ways. Losing scenes due to editing is nothing new, and in this era it is actually pretty neat we can access what would be buried in the a storeroom by some studios. In the past those two Star Wars scenes in particular were considered a "rumor" - didn't really exist. Release of SW on DVD proved they did.
And now you know, and knowing is half the battle.