Umbraco Macro Service exposes standard Umbraco Macros directly to the web for use in scripts or to provide direct data access to data using the already familiar Macro.
Similar to the Umbraco Base (and heavily borrowed against), Macro Service makes it easier to access your umbraco data for use in your javascript and other applications.
How?
Access your macro through the HttpModule (once configured).
Possible Usages:
Why?
Many non-developer types use macros to extract data and then use a single template to access this data through altTemplate style techniques. This provides a means to directly expose the data without needing an additional template.