Every website that displays user information on the front end will use profile fields such as first and last names for representing the members. By default Drupal shows only the username, which is definitely something you will want to change.
Modifying this is relatively simple. You could always choose which fields to use in Views, Rules and other modules, but the main problem is maintenance - the setup will be spread across many different pages and it's not the most optimal solution in the long run.
The right way is to change the way user display names are formatted by the system itself. This article will show you how to manage this for both Drupal 7 and Drupal 8 in your custom code.