I am a firm believer that challenging yourself to learn new skills fuels your work performance. However, I think work performance is only the tip of the iceberg.
It's a powerful force when you're able to do something that interests you. Letting your mind wander can produce or connect ideas or thoughts that you never thought you had. After all, there's a reason why Google, HP, 3M, and several other companies have adopted the 80/20 rule. The rule allows their employees to work on assigned tasks 80% of the time then spend the rest on tasks or hobbies they enjoy doing. The article can be found through the link below.
Make Your Job Feel Less Like Work with "20% Time"
As for myself, challenging yourself to learn new skills has an overwhelming effect on my overall health and well-being, which translates into both the quality and quantity of work performance.
Related: Hobbies Are Rich in Psychic Rewards