In the same way that SOA is a business approach and not a product, when you deploy an ESB, you are deploying an abstraction layer which provides connectivity and integration across your infrastructure. This addresses a very real business need to simplify the interaction between both existing applications and any new services and this applies to any business approach – SOA or not.
We find that Businesses often first deploy an ESB to address a specific need to connect and mediate between assets in one part of the business. Once an ESB is in place it enables other parts of the business to use or reuse the assets connected to the ESB. An ESB can be deployed purely for connectivity benefits but it often is also a logical step in a move to a comprehensive SOA initiative.