To keep your mind fresh and agile at all times, you should do a bit of mental restructuring from time to time.
Memories, emotions, dreams, and everything else that sits in our brain make us who we are. Without all of that "data" I wouldn't be me, and you wouldn't be you.
All of that "mental luggage" compounds over time; the older you get, the more memories you have. Our brains are well equiped for this, that's why healthy people are able to remember things from their childhood on their deathbed.
Yet, as we all know, keeping a clear mind all the time is incredibly difficult. Sometimes, strong negative emotions may "cloud over" our true self. Other times, when we're learning a lot of new information at once, it may be hard to understand it thoroughly in the beginning.
Here comes the concept of mental restructuring: giving yourself time & space to re-form thoughts, past memories, dreams about the future and everything else that is sitting in your brain. Of course, we can't physically enter the brain and move around stuff - even if it would be possible, it'd still probably be very dangerous.
By giving yourself uninterrupted time with zero to no distractions, surrounding yourself with something calming and beatiful - preferrably, nature, you get a chance to restructure yourself. To set things in a clear manner. Getting rid of the unnecessary, and revamping the useful.
No one thing survived without change. Even your own thoughts deserve that too.