The Purpose of Metrics
The purpose of gathering metrics is to understand the quality level of user documentation, to be able to improve that quality, and to be able to quantify that improvement.
By measuring quality, you know where you stand; you know if your quality level is changing; you can teach your quality techniques to others (success models); you can determine where competitors stand (competitive analysis); and you can determine if others are changing.
For a good overview of documentation metrics, consult the ISO standard on software documentation. (Sections of this area are taken from ISO/IEC JTC/SC7 WG2 Proposed Standard 26511.)