Dynamic accumulators at work.
i'm not sure grasses qualify as dynamic accumulators, and certainly that title doesn't fit while growing in a container.
a living mulch maybe?
dynamic accumulators are so called because they send down deep tap roots and access a deeper strata of soil, pulling nutrients up that are otherwise unaccessible to the neighboring plants.
as the dynamic accumulator plant cycles through the seasons, it drops leaves or dies or is consumed and redistributed via dung, and the nutrients which it accumulated during its life cycle are returned to the soil, enriching the upper layers.
the strong tap roots also open up hard-packed layers so that more delicate roots of other plants can access otherwise unavailable soil zones.
it has been suggested that dynamic accumulators can also share nutrients with other plants via endo-mycorrhizal associations with the roots of other plants.
dynamic accumulators include the common dandelion, stinging nettle, comfrey, dockweed, etc. Masanobu Fukuoka notes a specific tree in his book the one-straw revolution with a deep tap root that he plants in his orchards to act as a dynamic accumulator, the Morishima acacia (pp 63).
grasses are opportunistic plants, they move in quickly by focusing their growth laterally. that said, there are varieties which can establish roots as deep as 6 feet in the right conditions.
i don't know what type of grass you have there, but it seems to me that the main benefit would be from the enhanced soil structure and moisture retention inherent in a container full of healthy roots.
i have been experimenting with a "living mulch" of white, rose, and crimson clover, and rye and barley grass in my vegetable garden. unfortunately i had to abort the project after receiving a rather threatening letter from the head of my community garden. oh well, gotta play by the rules if you're leasing i guess!