Simply put, the more clutter you can see, the more easily you find yourself distracted.
Decluttering can mentally prepare us for other activities. Such as:
It clears the physical space so we can partake in the activity.
It provides us with a sense of accomplishment which translates into motivation.
It gives us time to think about or organize the activity in which we are about to engage.
When it comes to the mind and body, we can think of those two systems as being independent, they are actually two sides of the same coin and cannot be separated. If one part (say, the body) is agitated, then the other part (the mind) will follow. We are not stressed or agitated by occurrences themselves, but by whatever special significance we give them. Hence, when we have a clear space, a clear mind follows.
Note that an organized space can help set you up for success but it’s not the key to creative bliss.