I think of things I need to do when I’m at work, or in the car, but by the time i get home, play with the kids, and have dinner, I've totally forgotten what I needed to, or wanted to do. Its the biggest reason I got behind in HTTS. I simply forgot to check in on the classes for days at a time. I'd remember at work or on the way home, but by the time I could actually do something, I'd forget.
One of my favorite things about RTM is how intuitive the site is. When you set up your tasks, you can put in a due date of the date say June 30, 2011 or 6/30/2011. It understands it either way. But you can also put in due dates like “next Saturday” or in “two weeks”.
You can do the same kind of things with repeat. For example, the program understands what “Repeat every Tuesday” means. It even understands if you put your due date in the subject. Entering “party tomorrow at 9pm” will set your task as “party” with a due date of tomorrow at 9pm.
I've used task managers before but then I'd forget to check the task manager. RTM helps me with this because they integrate into Gmail. When I get on the computer at night my browser opens to Facebook, my Yahoo mail and my Gmail/G+, there in all its glory is my RTM list.
They also have an Android app, but that requires a Pro account. I am planning on getting one soon because I feel like I should support them. They have made an excellent product.