There are a number of reasons for using CiviCRM on your website.
CiviCRM is powerful, it allows for email lists to be managed professionally with bounce management, groups, customsable templates and unsubscribe.
Additionally CiviCRM has a fully functional membership system enabling membership based organsations, website subscriptions and a range of other scenarios. For not-for-profits there are donation pages, print mail labels, organise events, present campaign pages and the ability to for individuals to raise funds for specific causes.
A very commonly used feature on sites that we build are web forms. Drupal has a highly capable system that allows people to build a web form by specifying the types of question items they want to use. Webform allows for emails to be sent and the results downloaded - in its basic form. There are many extensions.
Some uses of webform are: