Imagine you’re swimming in nice, warm water, happily making your own food without a care in the world (other than zooplankton). You just need to store up enough starch before nightfall to hold you through the night so you can quit swimming, absorb your flagella, and wait for the sun’s return. You see a green light below, and you swim toward it. You can’t help yourself; your phototactic machinery is hardwired to respond.
Next thing you know, you’re captured by a giant, tentacled polyp. You look for a way out, but there is none. You’re stuck there for the rest of your life, forced to work and have the food you produce stolen by your coral overlord. Resigned to your fate, you absorb your flagella and get down to photosynthesizing.