I think custom illustrations can make documents and interfaces infinitely more interesting, and that's one reason I like doing illustrations. I use drawing, painting, and the computer.
I have done quite a few designs just for the fun of getting something out in the world. T-shirts, stickers, rubber stamps, games, logos, coins, computer icons, or just something to fill an empty spot of paper!
Life is better with illustrations.
I have included a few illustrations on the samples page.