When we should call business layer through web service?

You can have all the core Logic inside  business layer except few methods that involve page control manipulation in presentation layer.

Usually its a best practice to call all methods via service layer for following scenario.

Suppose you  will create a new Mobile Application that will do basically the same operation that you do in your web application, so in this case you can directly call/use your service layer.

Again having a service layer you can expose some methods to third party developer/user to build their custom application.