Professional photographers who have a wealth of experience and have studied posing and the nuances of light reflecting and wrapping around the subjects make a big difference. I have shot more than a couple hundred weddings in the last decade and know the second a moment is right to click the shutter. Also gently posing my subjects in a flattering pose comes naturally at this point. Another point is how a photographer frames the subjects, watches the rule of thirds and compositions that are pleasing are considered.
I have 30 years experience in Photoshop retouching so can really enhance a good capture in post production.