How does a coalescent help latex paint form a film?

In the context of surface coating, a coalescent is a solvent added to latex paint to facilitate the formation of a continuous film when the polymer particles are in a rigid, glassy state—meaning they are below their glass transition temperature ($T_g$).1

The coalescent functions by plasticizing the polymer, which effectively lowers its $T_g$ and makes the particles more rubbery. This allows the suspended particles to flow together and coalesce due to surface-mediated forces once the paint is applied to a substrate. After the particles have fused into a solid film, the coalescent evaporates, leaving behind a stable, continuous coating.1