Why do you need a brain? Not all organisms have brains. Jellyfish don’t have brains. They do all right, floating about.
The main reason for having a brain is for coordinated movement. Sight, sound, and other sensory information is brought together; muscles are controlled to produce coordinated movements of the body in response to perception, to achieve the goals of the organism (e.g., eating, not getting eaten).
Organisms that don’t move don’t need centralised coordination of information. Trees don’t move and they have no brains, not even a nervous system. Jellyfish float, gently swim, and stay upright waiting for prey to hit their tentacles. They have a different type of nervous system for this less demanding repertoire, with separate sets of neurons to synchronise muscle actions in different parts of their bodies. There is no integration in a single central brain.
Even humans don’t always need their brains to move. Sometimes, your spine will do the thinking for you. Touch something hot, and in half a second, you will snatch your hand away. Temperature and danger receptors in the skin have rapidly signalled to the spine, where local neurons have already decided to trigger the withdrawal reflex, and the arm muscles spring into action. Quick, withdraw your hand! It is, as they would say these days, a real no-brainer. But anything beyond a reflex movement is going to involve the brain.
At the other end of the scale, human actions can be vastly subtle and sophisticated, aimed towards long-range goals. Studying for a university degree, for example. But it is worth remembering the primary design influence of the brain: make the right movement.
This influence is still discernible in our ‘high-level’ cognitive skills. Take attention. We focus attention on particular objects in our visual field or on particular sounds around us. The ‘attention network’ in the brain involves the circuits of the particular sense (sight, hearing) and two other regions – a system that processes space and a region that controls eye-movements. In the brain, attention isn’t some abstract part of thought: it is about orienting to objects in space and preparing to make the right movement – including movement of the eyes to look to that region of space to get more information. While ‘paying attention’ in the classroom may be held to be a high-level cognitive skill, inside the brain it is about planning for the right movement.