an important link in the food chain, because only medium-tiny creatures can efficiently eat super-tiny creatures
at first a way to keep nutrients in the top of the water column (floating poo pellets!) and slow the transport of organic carbon to the bottom of the ocean (which provides an explanation of some carbon isotope ratios we see at the base of the Cambrian)
also a way to rapidly transport nutrients from the top of the water to the bottom, where they could support a larger benthic ecosystem (concentrated food that hasn’t been totally chewed by bacteria on the way down)

I never did figure out exactly why he thought floating-poo plankton would have evolved before sinking-poo plankton - he never explicitly said anything about it, but he did advocate both floating and sinking poo as triggers for expanded ecological opportunity, and if you’re going to have both scenarios the sinking poo has to come last or you get the carbon isotope data wrong. But anyway, that’s the theory behind the plankton poo thing.