The default values for the accept header when used in the various data source types works in most cases. A user must only worry about them in few occasions. This way, you have a single URL for your resource, yet clients of a server can request what they need. In most cases one of the formats is preferred if no accept header is specified.
For example, for the following URL http://server/mydata/resource1, there are three ways to provide data, as an image, HTML, or XML. You can then use three different accept types such as image/jpeg, text/html, and application/xml. The advantage is that you have a single URL for your resource yet the clients of a server can request what they need. In most cases, one of the formats is preferred if no accept header is specified.
For the IBM® Rational® Quality Manager OSLC API, by default, if no accept header is specified, Rational Quality Manager returns the HTML representation of the resources. You must set the accept header to application/xml to get the XML data.