OR they can have bilateral symmetry. The bilateral symmetry arises when the animal has to move about in search of food, whereas the radial symmetry is common when the animal/organism attaches itself to one place or moves about very, very slowly such as starfish. There are some types of sponges which are asymmetrical but as far as I know they have the monopoly.
Symmetry appears to be the most simple and most efficient form for movement in nature, which we have emulated in cars, planes, wheelchairs etc. We wouldn't dream of getting on a plane with one big wing and one little one! I presume that a being who is physically balanced can move the fastest and be most agile, evading predators, and hunting successfully.
Its also worth noting that any symmetry is only approximate.
-- Updated February 17th, 2016, 1:58 pm to add the following --
I think I got my sessile lifeforms mixed up there! Radial is for some sessile animals like anemones and coral (not mussels), floating animals such as jellyfish and very slow moving organisms such as starfish. Bilateral symmetry is for more dynamic, free movement.