Polypainting allows painting on a model's surface without first assigning a texture map. A texture map can be created at a later time, and the painted surface can be transferred to the map. Polypainting offers significant advantages compared to standard workflow.
The resolution of the texture map need not be decided in advance. This is particularly valuable if you find you need more detailing on an area than you thought you would. Instead of repainting a new, larger texture map, you can simply transfer the existing surface painting to a new, larger map, with no rework necessary.
Similarly, the UV unwrapping need not be fixed in advance. If one unwrapping proves unsatisfactory, simply create a different unwrapping and transfer the surface painting to that map. Removing UVs from your model frees up system resources and allows you to work with even more polygons!
To understand how polypainting works, first consider a 2048 by 2048 texture map, which provides reasonable resolution. It has a total of a little over 4 million pixels.
If you work with a 4 million polygon model, then in terms of surface painting, simply assigning each polygon a uniform color gives the same amount of information as the 4 million pixel texture map. (Actually, somewhat more, since significant parts of texture maps are typically left blank.)
So, with polypainting, you can put all of the painting details directly onto the model's polygons, and then transfer that detail to a texture map when the painting is complete.