One of the reasons is that the original drawing is always done by hand - you just can´t beat pencil on paper. The other reason is that I use a library of watercolour textures when I paint. These were made by splashing, dabbing, spattering and just generally messing about with earthy watercolours on standard watercolour paper, leaving them to dry and then scanning them. By combining these in different layer styles (favourites being multiply and overlay) and opacities in Photoshop one can achieve quite interesting results.
Here´s a few of these textures for your perusal. I´ll be back shortly with a more in-depth tutorial on my work process, but until then feel free to download these and try them out yourselves. Of course they´re all clickable for larger versions.