Both men and women do this. Are we expecting something else to be there? 

One reason is narcissism. People who find themselves physically attractive in any way like to look at themselves in the mirror. They enjoy admiring their own muscular development, even if they're not as developed as they think they are. A lot of people also do it after a workout set to see how good a "pump" they get. Others who work out do it, because they're anxious to see progress. There may be other reasons, but I believe it's a combination of the three I've given.

Looking at their muscles while flexing is a way to show that they are proud. Small bits of progress are still accomplishments. Then when it comes to bodybuilders they are looking to see if their muscles are developing. Using a mirror is a means of seeing symmetry and conditioning. So doing poses in a mirror is practice for a competition.