This service is used between the VLR and the HLR to cause the HLR to mark its data for an MS so that any request for routing information for a mobile terminated call or a mobile terminated short message will be treated as if the MS is not reachable. It is invoked when the subscriber record for the MS is to be deleted in the VLR, either by MMI interaction or automatically, e.g. because the MS has been inactive for several days.
(excerpt from 3GPP.29.002)
手机在VLR里面长时间没有和网络联系，VLR要向HLR通知，发出“Purge MS"信令，HLR会返回“Purge MS（Result）"！
主要是解决MS的被叫问题，减少了HLR向VLR要漫游号的过程，HLR会直接返回信息！
(excerpt from http://mscbsc.c114.net/viewthread.php?tid=16473)