First lets know that what is Doubleclick?
Lets know it with a simple example that when a company seels or buy anything it is not neccessary that the sold or bought things also delivered by the same company.It can be deliver by any company also. That seeler or buyer company is google (Adsense,Adwards) and the company that will provide services to deliver or make the services better is doubleclick.
By this it will be much easier for google to reach its all users (Publishers and adwards).This service by google will also a great step to make its other services better.