A big contribuiton to the quality of the images and to the overall experience is given by a good make up artist / hairstylist
Having somebody reaching the client in her hotel and taking care of make up and hairs in a way that will look good in images (this is slightly different from usual make up) gives a hust boost to image quality (and saves post production time and costs).
A second contribution is the the overall experience: having somebody taking care of the client's beauty helps a lot being relaxed and in a good mood.,
During large weddings a make up artist is always present, I reccoment to consider one even for elopements and for prewdding or trash the dress sessions.
During engagement and portraits it can be useful but brilliant results can be achieved even without (expecialy with young couples).
I typicaslly provide a list of endorsed professional to clients that book my services as it is important the besides the professional qualities there is a match in style.