The monirail first opened in DL way back sometime during the 60s, I believe. There was no real "practical" use for the thing...it was basically just an attraction in its own right. It was something new and attractive, especially for Tomorrowland. It would also be used as a transportation system to the Disneyland Hotel.
Walt loved progress and future technology, and trains! Put those together and you get the monorail. I would think that Walt was also using it as a living showcase to the public as a model for what could be used in cities all around the U.S. The monorail was a large part of Walt's idea for Disneyworld and Epcot.
Now, of course, there is no real need for the monorail. Even if you're staying at the DL Hotel, it would be faster for you to just walk to the Park. People still use it, of course, because it is pretty cool. I rode it when I was there last year just for the scenic tour
. They obviously have no plans to get rid of it, I am guessing, because they added that pass over the DCA Park, which I think was a nice touch, and a nice update to the old Monorail. There is plenty to see from the monorail, so it is still a nice attraction.