I also coded websites by hand and had a bunch of functions built that I could pull into new websites (functions for database abstraction, cleaning user input, etc). What interested me about Drupal is not just all the abstraction (which I already had) but the security.

I personally do not have time to make sure that I deal with every conceivable type of attack possible - Drupal, as a community, can deal with new attacks quicker than a single individual can.

After developing in Drupal, I'm now also very impressed with the abstractions as well.

Using a CMS like Drupal can be enhanced by people with programming skills.

So you know where I'm coming from... I've written my own framework, cms, and sets of libraries in more than a couple languages.

To break this into 2 parts....

Why a framework/cms

After writing my own stuff I realized I had to support that. If bugs needed to be fixed it was on me to do it. When I wanted new features it was up to me to write them. For a standard client website that's a lot of work if they want anything beyond the basics.

Enter frameworks and content management systems, especially the open source kind. The popular projects have code that's been beaten with a security stick, there are others working on features alongside you, and the tools to build the site are not just on you to create.

Additionally, if you're tool does your CRUD and low level stuff well and there are others helping with that stuff you have more time to concentrate on the fun stuff.

Oh, and if the project is good there are good people around you to learn from... which helps you improve your skill.

So, that's why I'm a big fan of using a framework or CMS. If you're doing a large scale project, like facebook, it makes sense to write it from the ground up. For most people it just doesn't.

Why Drupal?

First, drupal has a great community. They are people you can teach and learn from. People you can struggle with. People who are just fun to work on a project with. Your 'co-workers' matter and drupal has a great community of people to work on stuff with.

Second, drupal is a CMS that acts like a framework. If you're content centered drupal is a great choice. If you're moving a way from content into other things it's not the best tool. The framework aspect it really powerful and design patterns have been implemented in really useful ways that aren't common. The API is well documented and it's always open to change, as long as that change is for improvement.

Not to mention that Dries, the project lead, has a PhD in computer science and really knows his stuff.

Third, drupal is a great selling point. If you're trying to pimp yourself drupal is one of the hot kids at the moment. With the way it's willing to adapt it fights being stagnant as well. Drupal has been written up in business magazines, it's one of the golden tools in non-profits, and it's used all over which makes it good to sell for projects (i.e., get work for yourself).

Fourth, drupal is looking to push the user experience envelop with goodies. That's good for clients. They love the wow factor. Check out http://d7ux.org.

If you don't go with drupal (and there are lots of good tools out there) I would suggest grabbing a framework or other development toolkit to help you out.