If you cant draw anything at least fairly good in photoshop with your mouse/external canvas, please don't start to think that tablets automatically increase your drawing level. People who are talented with it came a very long way from where you are right now.
As john said, tablets won't necessarily increase your drawing level. But it more closely emulates physical drawing than with a mouse. The benefits with a tablet, I would say, is that you'll be able to draw in a variety of different programs as a digital artist. There's an undo /redo button so you can erase absolutely anything (something that's hard in real life). Overall, there's a lot of fluidness, and speed that digipainting offers that real canvas work doesn't. A lot of this is due to the fact that you can save any state and load at any time. So in essense, you can explore several different themes and steer your artwork in different directions. If you don't like it, load up and try something else. i guess it's more of an ease of access thing. ;=;
Pretty much, the added features on the Intuos and the Bamboo make for easier navigation as well as increased speed while drawing. But if you have no knowledge of anything, it won't help you in the least.