Drupal's just another content management system. And it's all well and good for a developer to write their own, provided they have the time and skill to do so. Pre-made content management systems are particularly good for either the people who don't have the skill-sets to create a custom system, or people who don't care enough to create a custom system and just need something up right away.
Of course, Googling, or even taking a random guess - drupal.com - would have given you probably that much information.