When inspiration strikes you want to move as quickly as possible, dragging that mouse all over the screen can drain you of your creative juices. Learning these Photoshop shortcuts will help you be more productive and more focused on your designs not the tools.
Knowing my shortcuts in Photoshop is probably the most important thing to me and probably to any professional artist! It definitely has increased my work output rate a lot by cutting down on my time looking for these tools. Most of these tools are used in 90% of the artworks that I do on a daily basis.