The particle to grid emitter takes care of particle splatting. This simply means it encodes the data from a particle (lets say its color property value) into an Effex channel grid (for a color property this would require a vector channel).
So each grid cell that is spatially near a particle can get information from this particle. This allows to customly fill Effex grid channels and use these for variable viscosity, variable density, grid constraints but also to render particles with the gas renderer.