Since the standard output template returns all attributes, even for empty elements in the template, you might want to tailor information to your specific business needs. If you don't exclude the attributes you don't require, you receive more data than you need and the extra data may slow the performance of the API.
For example, if you are using the getOrderDetails()API to return only OrderLine attributes but your custom output template includes Schedule attributes, all attributes for OrderLine and Schedule are returned.