One challenge the Drupal community has faced for some time is a labor shortage. There are, quite simply, not enough skilled Drupal developers to go around. That's quite a problem when the Drupal market is continuing to grow steadily.
In a Drupal site, if we have any module upgrade or configuration related work to be done in our live site, it is recommended to put the site in maintenance mode, so we can prevent end users from experiencing glitches.