It shapes (squishes) the extrusion so that it bonds better to the surface beneath and adjacent perimeters and infill. This improves adhesion and the strength of the print.

It also is critical for getting a good first layer.

Cross section of the extruded filament without anything otherwise affecting it would be circular and roughly the diameter of the nozzle size.

Extruded lines of filament with circular cross sections only just tangentially touch one another, providing poor adhesion. When squished down into more rectangular shaped side cross sections it has more surface contact with the plate (or layer) below and the adjoining walls and infill.

More surface contact in the extrusions creates better bonding and increases part strength.

It's critical for getting a good first layer because the extruded filament needs to be smushed down into the plate just the right amount to shape the cross section so that it is firmly snug with the plate to stick in the textured PEI plate, while not being too close that it over extrudes along the edges causing the infill to buckle, and while also not being too high so it only lays on the plate.