WebService Interface

[Shankar sanjay]

Hi, I have red "Mikalai Zaikin" study notes in which he has given statement like

None – If not specified, the endpoint implementation class
is used to generate the web service contract. In this case,
a service endpoint interface is not required.

If we have not defined WebService Interface then JAX-WS will generate by default and lot of example he has given webservice interface instead of SE Implementation class

Can anyone explain, when we have to define webservice interface and when we should not