It depends on your personality. Some can wing it and don't care if they see everything. They just want a family vacation. If, on the other hand, you want to "do Disney," and you need to get on Frozen, Soarin', and Test Track on your one Epcot day, that takes planning and knowledge of the entirety of the system. When do I wake up? How long should I plan for transport? What order of rides? Which FP do I choose? You can't wing that.
And then you want to eat at Le Cellier because your friend Cindy said the $60 steak is better than Ruth's Chris?
And the next day you are doing Jedi Training for your kids Liam and Gavin who you've been promising this to for the entire 342 days of your "countdown to Disney?" And Friday you've promised your daughter Skylar three hours of pampering in the Bibbiti Bobbity Boutique IN THE CASTLE followed by lunch in the Castle while Dad Tim takes Liam and Gavin to the Fort Wilderness Lodge for horseback riding, archery, and lunch at Whispering Canyon (let's hope Tim realizes Mom got a little confused while highlighting Birnbaum's). Then we will all meet for dinner at Be Our Guest.
It takes a lot of planning and despite what some would suggest, the fact that many folks have some MUST-DOs that ARE hard to book last-minute requires that they plan EVERYTHING. If you need to know when you'll be eating in the Castle, going to BBB, eating at Be Our Guest, and eating at Le Cellier many months out, and when you'll be taking the family on Frozen, Flight of Passage, and Mine Train 60+10 days out, you also need to know when you'll be everywhere else.
It's easy to forget this when we go frequently and don't feel like we have to see, eat, buy, and do everything.