Cell communication is vital for cells to know that to do. By secreting molecular signals in the extracellular environment microorganisms can indirectly assess the cell density and respond in accordance.
Communication works through signal transduction pathways and conserved through evolution. The Signa transduction pathway for single celled organisms and multicellular organisms are almost identical. This could mean that they have been around for a long time and have been co-opted for different things.
Single celled Organisms
In single celled organisms it is a response to the enviorment.
Multicellular Organisms
In Multicelluar organisms, they coordinate activities