I guess that in intuitive terms, being tired just means your brain needs rest to regain its strength after having worked for a long time.
I’m going to speculate that this need for recovery could mean that everyday though puts stress on the brain that does minor damage that needs to be rebuilt (much like using a muscle a lot causes tiny rips that are restored gradually, which is how muscles grow), or maybe it depletes some sort of resource (i.e. nutrients) that the brain then needs to gather again in sufficient amounts to be able to function properly again. Or it could be the opposite; that brain activity produces some sort of waste product that needs to be gotten rid of. Or a combination of those things.
Or maybe there’s some other explanation that hasn’t occurred to me yet.