I almost always do web development with a simple editor like Geany or Notepad++. Recently I'm starting with Aptana, and I'm a bit confused about the purpose/benefits of the workspace.
Why is it useful? How do I use it?
The Workspace holds the development environment.
Each workspace houses a collection of projects, so it was determined that creating multiple workspaces was a good way to get organized. Each workspace stores the perspective and views for that workspace.
In my opinion you should have a new workspace for every new project you work on. You should also have a testing workspace in which you should have all testing projects. Another benefit is that when you switch your workspace for a particular project you get all perspectives and views you have organized for that specific project. You don't need to worry about files of all projects if you have separate workspaces for them, as you have files that are related to that project only. This way it also speeds up building/compiling files in workspace.
Another similar kind of discussion, but for Eclipse: Benefits of creating a new workspace for each project in Eclipse?