If you have a brain, you carry an inner reproduction of the outside world. Your body is also represented in facsimile inside your skull. Among other things your personal inner facsimile lets you try things out inside the confines of your own head, to rehearse scenarios without worrying they will kill you. Inner imagery allows for repetition of events in imagination. It leads to the consolidation of memory and improvement of performance, the acquisition of knowledge. Emotions also play a prominent role for the dreamer allowing processing of fear or increase of avoidance that colors experience and enhances motivation. You are also free to save inner events in memory to compare to real experiences or talk with fellow travelers later on. You may invent your own applications, alter contingencies plan, for future performance the full imaginary capacity at your command.
Animals without a brain can do none of this. That doesn’t mean they are not conscious or aren’t struggling to stay alive. I don’t count a central nervous system as necessary for consciousness. Even protozoa like amoebas have been caught in the act of a struggle to stay alive as predators and prey, carrying out strategies to preserve their own lives but with no brain. When you see them strategizing and laboring to avoid death, it makes you wonder if they too are afraid, like us, to die and if these single cell organisms might even experience pain, struggling as they are against their own destruction. But there is no evidence these creatures harbor any kind of internal model of the world. Since they need to adapt and survive in the real world their experience is more intimate than with those who harbor an inner model of world and live in the world. Having a brain allows for less immediacy or intimacy with the outside world the ability to manipulate events and contingencies in your own mind, at one remove, and without real consequence. Brains also allow for constructions of worlds of complete fancy bearing little relation to reality. So you can view the brain as a tool that for a time allows us to step back from the real world and even our body. I will explain as we go along.