When excessive amounts of nutrients end up in streams, why should we care?
Because they cause eutrophication.
Excessive nutrients end up in streams, lakes, ponds and cause algal blooms. This limit oxygen transfer into water column. Furthermore, sunlight cannot penetrate in deeper parts due to algae growth on the surface. Anoxic/anaerobic decomposition might occur in deeper layers of water column and this is unbearable time to time.