Personally, I kinda like the fact that we do not know about dreams and sleep. Like there are theories but the most we know for certain is that if we don't sleep, our body stops working correctly and we die, as well as the fact that REM (the period in which you dream) is super important and your body will rush straight to that if you spend a while without dreaming.

Some of the main theories (ignoring spiritual ones, obviously,) are:

1. Sleep is how we transfer memories to long term, and dreams are just how our brain interprets those thoughts in that time.
2. Our dreams are our brain's way of preparing ourselves for different scenarios irl, which explains why a lot of them are stressful.
3. Our brain has more energy to devote to complex thought because we aren't using the rest of our bodies.
4. Random hallucinations quite frankly.

I'm not a scientist but I've done some research into this stuff, because I find it interesting. Hope this helps!