REST is a very powerful way to build services. The model lends itself to high scalability as it uses the same model as the web. It is particularly applicable to services that are exposed on to the public internet as generally all a consumer needs to be able to do is manipulate XML and talk HTTP – which every platform supports.

You are able to compare why web service? Why REST under the following link.

http://www.developerfusion.com/article/7991/rest-and-net-35-part-1-why-rest-based-services/