The default values for the accept header when used in the various data source types works in most cases. In most cases, one of the formats is preferred even if no accept header is specified. A user must change the value only in few occasions. Changing the value enables you to use a single URL for your resource and request only the data format needed.
For example, the following URL http://server/mydata/resource1 contains the following data types: images, HTML files, and XML files. You can specify image/jpeg as the accept header value so that only images are accessed from that URL. The advantage is that you have a single URL for your resource, yet the clients of a server can request what they need.