XMP does indeed map values to exif tags.
Both EXIF and XMP are rich sources of image metadata.
The benefit of XMP is that it is easily readable and any XMP-aware application (like all Adobe products) can manipulate these properties.
XMP derives the values from the native values in the image, so basically XMP provides a mapping between properties in the exif block in images and reconciles these values in the XMP namespaces as defined in the ADOBE XMP SDK documentation.
The benefit of using the XMP SDK to manipulate metadata is that then the responsibility of reconciling between different image metadata formats (Like exif, IPTC or XMP) while reading or writing is transferred to the XMP SDK.
If any change is made to the XMP property, it is reflected back to the exif block in image. Similarly if any non-XMP aware application has modified the exif metadata without modifying the corresponding xmp value, at the time of read operation, the XMP SDK will reconcile this change into the XMP value and while writing, this change will be saved back.
Using the XMP SDK is to manipulate metadata is basically easier as you can leave a lot of format specific detailing upto the SDK to handle.
More information on different sources of image metadata is available on the Metadata Working Group.
The complete Spec can be downloaded from here.