Building Custom Service Applications But When?

1)Whenever there is requirement to share the data across cross sitecollections or across farms

 or if you want to associate third party service may be for analytics or computation. building custom service application is must

2)To do so, you need to develop Webfront end server (Proxy UI for configuration), Application server (WCF end point) and SQL databases

Checkout this to get standard template to build custom service application

http://www.lifeonplanetgroove.com/introducing-the-sharepoint-service-application-starter-kit/