Other creative pursuits are a way to help focus the writing mind by giving it time to rest.
They also allow us to see our own artistic progression in another medium. If we begin to draw every day, keeping a record, we will see growth.
Writing is a different beast.
It is hard to see growth. Longer written works such as novels or screenplays take a long time to develop. When a project takes upwards of a year of daily work we become blind to our own progression. Only when we have let the project rest and gained perspective do we see some form of our own improvement.
Even then our view is skewed.
If writing is our main creative pursuit then finding another is a sure way to become accustomed with the hard to accept truth that we do, in fact, get better.