Some idea of which rides are likely to have very long waits and run out of Fastpasses may save you from disappointment. Similarly having a rough idea of crowd patterns helps, too.
For example, it's good to know that most people will arrive at Epcot between 10 and 11 in the morning. If you head straight for Spaceship Earth along with everyone else, you'll face much longer waits for the ride than if you try later in the day. So being able to tell the family, "Don't worry, we'll come back for that later," is very helpful.
On the other hand, if you leave a ride like Test Track for later in the day, you're likely to find waits of 90 minutes all the Fastpasses long gone. The lines for that one are only short at park opening. So get over there early to grab your Fastpasses.
A Touring Plan basically saves you the trouble of learning the ins and outs of all the rides. If you follow it, you'll be avoiding long waits in line.
Do you need one to have a great time? No, absolutely not. But I'd still recommend reading through Josh's Cheat Sheets, just to familiarize yourself with things.
http://www.easywdw.com/category/cheat-sheets/
It's not like it's a terrible chore to read up on the fun you'll be having at Disney, after all!