Gee whiz ... this is a tough one, in that, all of us have different reasons and motivations for implementing something like GTD. My wife and I both use it, but came to it with many different contributing factors as personal motivators. That being said, I'll take a shot at it from the vantage point of how we BOTH benefit from it.
First, it creates a solid system for handling the intense flow of responsibilities, desires, and dreams that we all have in this day and age. The voicemails, cell phones, faxes, emails, kids, boss(es), vacations, in laws, paper mail, etc etc etc just piles up ... UNLESS there is a way to process it all. GTD is just that, a system for processing the 'stuff' of life in such a way that you can do what needs to be done. Putting things in each others' in boxes has saved our marriage dozens of miscues and miscommunications ... if not more.
Second, it works against the human tendency to procrastinate on tasks that we dread. Notice I didn't say eliminate . Since the dread factor is increased by the size of the thing that needs to be done, breaking the thing into doable next actions is wonderfully motivating. My bills haven't piled up once since GTD ... and that's after 13 years of monthly piles of bills, insurance papers, etc. I literally have not one pile in my life, while I used to have several at home and work, and in my car for that matter.
Third, ALL the most reliable and productive people I have ever met use a system like GTD. Most of them don't call it GTD, and haven't learned it from DA, but when I ask them how the do it all so well, they will explain some kind of system for processing, remembering, and completing tasks. Some of them learned from their parents, others from a mentor or teacher, some from sales training, others just thought it up ... but I have yet to find a productive and reliable person that doesn't use some sort of GTD like system.
For me, I read GTD on a whim at Barnes and Nobles, just because my secretary had recently subtly suggested I change my methodology somehow. I noticed the book, turned to the chapter on procrastination and thought that DA had a video camera set up in my office. It described me (and most people I know) almost exactly. So, I read the rest of the book and implemented it within a week. That was over a year ago ... and I am very thankful for it. Honestly ... words can't express how much this has helped me, my family, my peers.
Why GTD? Platform-Independent, Open Source Time Management
On one of the GTD Fast tapes, David Allen says something like "this is not some new age stuff, it's just methodology." Practical, bottom-up, advice that makes sense, works well. You don't need to buy stuff from DA's company, you don't even have to read the GTD book (but it helps). You can layer whatvever scheme you like on top of your projects and next actions. If you like "governing values" (pre-Covey Franlin) or "Roles" (Covey), or whatever, no problem. Make your list and review it as often as you need to. Honestly, though, I have found that David Allen's characterization of this stuff in terms of altitude (0K- next actions; 10k- projects; 20K- current responsibilities, et cetera) is a very good way to characterize higher-level goals et cetera. I used to buy a lot of books on time management, life management, the whole shtick. Since reading GTD, the only other book I've bought on those topics is Ready For Anything, Allen's 2nd book. I really feel no need for anything else. And you can implement the process in so many ways. It is platform-independent, to borrow some computer jargon.