I found DnD was a phenomenal vehicle for learning to draw. In the early days when you start drawing, the most important things to progress are (in approximate order of importance)

1. To draw a lot.
2. To draw regularly.
3. To spend time looking at art you like, to study it - so you can try and work on making your art more like it.
4. To have friends look at your work and give you constructive feedback.

One of the hardest things with drawing a lot, and regularly, is having inspiration for material for "things to draw" - and DnD provides this in spades - regular new events that make for dynamic imagery that you can try to capture week-on-week... as well as a group of invested friends who want to look at your art.

When I started playing 4e, I started drawing pictures of the characters each week, and we went from this:

To this, in about a year: