Spring DAO interface/implementation structure

[Osborn Tamara I Contractor 10 CS/SCA]

I'm a newbie so please forgive this general question. I am testing an
implementation of iBATIS with Spring DAOs using the interface/implements
pattern. I was asked what purpose the interfaces serve since I have a 1
implementation for each interface. Every example I have seen has this
structure but I haven't found documentation explaining why. Can anyone
give me information or references so I can research this? 
 
I appreciate your help.
 
Tammy Osborn