Contact pages contain all the information needed for people to get in touch with you and they allow you to centralize the contact information you have on your site.
having contact details all over your site makes it very difficult to manage, and when things change you find yourself running through hundreds of pages looking for email addresses and telephone numbers.
also you have no clear idea of how many people are coming to your site just to read the phone number.
By using contact pages you have a central place for all your contacts and you can start to measure who is contacting you and from what page.
Using Contact Pages on content pages
For content pages there is a contact setting on the page info tab that lets you define a contact for the page. when you set this a set of links are rendered on the page. which the user can then click to get to the contact page.