How is radial symmetry advantageous to sedentary organisms?
Radial symmetry is advantageous to sedentary organisms because sensory receptors are evenly distributed around the body.
Also, when you imagine sedentary organism what can you conclude?
For sessile (sedentary) organisms, radial symmetry is useful because the animal can just "sit down" and grab food or detect threats from all directions.
One organism from Croatian sea