The developers tool provides the user with quick access to all entity metadata in CRM. You can access this information by going into the customization area of CRM, this will provide you with most information but this requires lots of clicking. This tool is purpose build for accessing meta data and does just that.
This tool also provides you with access to more information, you can view the SDK messages associated with the entity. For SDK messages the availablity and in and output parameters are also shown.