Strictly speaking, it's not necessary; Disney transportation will take you anywhere you want to go on-site.
However, I always rent a car at WDW. Driving myself saves me a lot of time each day, particularly if I'm going from one resort to another, since there is no direct transportation from resort to resort.
But even a simple trip like a bus ride from Pop Century to Pleasure Island can take up to 50 minutes, while it takes only 15 minutes by car.
Having your own car also means that you ALWAYS get a seat, NEVER have to wait for another bus to come along if one gets full before you get on, don't have to listen to other peoples' conversations and phone calls, or bask in the aromas of folks whose deodorants quit (four days ago) in the Orlando heat.
It also means going where you want, when you want, any time of day or night, on-site or off-site. You can go off-site for shopping, groceries, dining, photo downloads, or to visit other attractions like Universal or SeaWorld.
Since I've never been a person who feels that driving myself is "SUCH a chore" and needs to be driven around all the time, I prefer to control my own destiny, save up to an hour per day of my precious, limited vacation time, free myself from bus schedules, and never have to stand up going from one place to another.
I also don't miss the cheerleaders singing "Rudolph the Red-Nosed Reindeer REINDEER!!!" on the bus at the tops of their lungs during the Pop Warner tournament in December.