From what I've read, the brain partially 'compartmentalizes' it's communications at regular intervals, and dreaming is what happens when this process finishes:

From what I've read, the brain partially 'compartmentalizes' it's communications at regular intervals, and dreaming is what happens when this process finishes: the different compartments start communicating, synthesizing multiple independent thought processes into a cohesive whole. (Which would explain why they can be seemingly random and also relevant of recent events.)