Whereas marram grass binds sand together so common cord-grass (Spartina anglica) binds mud and silt together and starts to create saltmarsh. At the western end of Poole Harbour where the water is more subdued and there is less wave action than on the exposed shores that are subject to faster tidal currents, silt from the various rivers that flow in to the harbour settle and create mud. This mud is covered by salt water for around 12 hours a day and uncovered the rest of the time Anything growing here has to be able to withstand these changing conditions and be resistant to salt water.
As with marram on sand so cord-grass can cope with unstable mud. It is a very successful plant of the shore line mud flats and, in some places it has been planted to specifically achieve stability in these conditions.
Perhaps not an attractive plant but an essential part of the process where vegetation recolonises lost ground.