If nothing ever eroded into the water, then there would be very few nutrients in the water for other things to use to grow, and in turn be food for other organisms in the food chain, and all the way up the line.
Also, if there was no erosion, then we would not have the large deltas/wetlands at the mouths of rivers, in which many animals live. (these are formed from depositing silt which had been eroded from farther up the river)
