This is a diagram of the Glymphatic system. The Glymphatic system helps to clean the brain of it's waste products while we sleep. It could be that this is the main reason why we need to sleep.
The short version of how the brain clears out waste when we sleep.
Dr. Maiken Nedergaards explains the glympatic system in more detail in this great presentation.
It was only recently discovered that lymphatic vessels do go into the brain.
The video above is a bit dry. This one is a bit more entertaining.