Autoresponders and "Drip" Campaigns
Again, the primary purpose behind marketing is to create leads.  If a potential customer goes to your website and does not have a number of ways to submit their contact information (email address or phone number)...you have failed to get a lead!  One of the best ways to collect names and contact information is to offer them something of great enough value that they will be willing to take the Spam-A-Lot challenge!

Autoresponders are a series of emails that are design to keep the prospect interested and informed about the value your service or product can bring. The first autoresponder which is sent right away is a thank you for your inquiry email.  The rest are sequentially sent in the future to make sure that you do not lose touch with a potential customer.  

An autoresponder can also be used to create a "drip" campaign, which is a sequential email campaign which presents your message in small messages across a period of time.