One of the most powerful reasons for combining Web Content Management Systems (CMS) and CRM is the integration of web forms with "back end" data.  Web forms are great for entering data but the real power comes from a full integration with CRM data without having to write any code.  Drupal and CiviCRM are integrated through the use of a special Drupal module that provides "out of the box" integration.

Imagine creating a web form by just clicking on check boxes to add fields that are already defined in your CRM database!   Here is what that looks like:

The webform is automatically setup - with many options for styling and layout.  And the form automatically uses the design theme for the website.