that's about the same as asking: 'why having applications while we have classes, interfaces, ....'
a service can be seen as a group of classes/interfaces/... bundled together to perform certain tasks and provide certain functionalities that can be used by an unlimited amount of applications.
google's gmail is a service. those who have an account, are the 'applications' using it.
I don't know why jwenting wrote what he did - but whatever he wrote is his personal opinion and doesn't necessarily represent DaniWeb official policy.
Of course it's OK for you to post your questions here.
If that was a question you were asked for "homework", then our rules say you should show some effort first, but if it's just general curiosity, or a desire to know more about OO and Java, then even that doesn't apply.
Hopefully stultuske has answered your question, but if you need any further development of the answer please ask.
you can post questions, but your "question" indicates you've not done any work of your own to find your own answers.
It's also stereotypical of homework kiddos, not doing any of your own research, not putting in any effort, and dumping their homework piecemeal, poorly reworded to make it seem like it's not copied verbatim from some book.