This link is a great video of someone panting a face in oil.
My question happens about 3 minutes and 25 seconds into the video.
The face is 75% complete and the artist takes, what looks like a large paint brush (like for walls) and he/she passes it twice over the face to "blend"? the colors?
Has anyone done this? Seems like it would cause more harm then good??

David
Comments
This is an effective way to begin to bring in overall color harmony into the entire painting. Colors of the face and hair often reflect colors from the environment that the model is sitting in. And I like the texture that this technique offers as well.
This is quite effective, I like it enough to try it sometime soon. I would first practise this technique on the side before committing it to a painting in progress. Big brushes like that are fun to use, a variety of big brushes bring unique textures and effects into a painting. I'm just new to these big brushes and I look forward to using them whenever I can. Ordinarily these brushes use up a lot of paint, but the above artist's technique of dragging down what is already on the canvas is economical, I like that too.
Denis
Done in the earlier stage of painting it blends and as @forgivness says harmonises.
Also, as @ArtGal mentioned it makes the surface smooth. This is useful if you don't want bumpy paint layers.
I also do this on edges between subject and background, think it helps prevent your subject looking cut out/stuck on.
While it can look like you have just smudged everything, you refine at the later stages.