Under the NYC Green Infrastructure Plan, bioretention facilities, like the stormwater Greenstreet above (and smaller streetside bioswales as well), are being built all around town in areas served by combined sewers. Their primary purpose is to capture stormwater runoff, reducing the amount that drains into the sewers and thus reducing the volume of sewage that overflows into surrounding waterways during times of significant rainfall or snowmelt.
On a related note, I finally figured out what this thing is.
