When working with web services, most people start by consuming a service that is offered by somebody else. There are two different reasons why you might want to consume a web service:
1. You need to access customer data that cannot be accessed just by sending queries to the database. The reason for this might be security or the use of a data source not supported by PHP. Often the reason might be that you also want to access business logic that somebody else in your company has already implemented in Java, for example.
2. You want to use a service provided by another company. For example, if you want to integrate a search into your website, why would you bother writing a new search engine, if you could just as well use the search service offered by Google and pay for using this service. It will probably still be cheaper than implementing all the features Google has to offer. The same applies if you want to build an online auction, sell books, etc. There already are...