An animal might be more likely to develop an asymmetric body plan if it evolved in a place where events consistently occur from the same direction.
I'll explain more, but first an obvious example:
Feet. Humans are bilaterally symmetric, but ignoring which way a human is supposed to be oriented we can see that humans have special adaptation for gravity. Gravity, for our evolution, was always one direction. Because of that, it was useful for us to develop some motility appendages in that direction. That asymmetrical adaptation is so obvious we usually don't see it as asymmetry.
Where this can be found
Ok, so back to gravity being down, what about horizontal directionality? This is less common, and with only a few exceptions, is likely the reason most animals on Earth are symmetric in this plane.
However, it's not hard to imagine a world, or at least a place with specific conditions on that world were horizontal symmetry is not as advantageous.
For example, consider a tidally locked planet, on this planet there is a thin terminator where life exists and evolved. There are a few natural conditions with consistent direction: gravity is always down, in one direction it's always light, the other is always dark, the winds would also likely have a constant direction depending on latitude. A sessile creature evolving there may only look for food in the direction of the oncoming wind, defecate in the opposite direction, absorb energy from the sun side, and dissipate heat on the dark side.
This isn't too far fetched an example. It could be something as simple as a primary predator or competitor that always attacks from the left in face to face battle. A consistent ocean current on a reef or wind pattern in a valley. Anything where the direction of a natural force is consistent for enough time to evolve an adaptation for it.