[dba-VB] Why use a contract?

The WCF book I am reading discusses building interfaces / contracts.  He uses the term 
interchangeably.  Basically you set up a file that has just the variables and method signatures, and 
name it.  Then you build an actual implementation class that implements the contract.  I am the kind 
of person that likes to know the "why".  I don't mind doing something but I find it useful to know 
the reasons for what I do.

So what does the interface / contract do for us in the development process.

-- 
John W. Colby
www.ColbyConsulting.com