>>12070201Bilaterel symmetry allows specialization of segments (arms sprout from our upper torse, legs from bottom of torso, etc). Symmetry across the central (sagittal) plane is fairly easy, it's fundementally the same genetic code and embryonic development on both sides, and it's preferable(locomation requires the body to be balanced so it doesn't fall over on spin round in circles). Bilateral symmetry also gives 1 direction to the organism (for sensory organs and locomotion) which is much more neurologically easy to handle when wanting to go a definite and intentional direction (which is useful in hunting, where the goal is not just ubiquitously available in the surrounding media).
Some organism practice radial symmetry (like jellyfish) where they are symmetrical across every plane that passes through their centre. This doesn't allow easy specialisation though, and is very ineficient for more complex structers. Imagine a human that was just legs in a big circle out from the centre. If all walked at once it would go nowhere, and if it stood 'up' so the legs were all of the ground exept for two it would be very hard to balance and a massive waste of resources.
