As you fight monsters, if your HP meter turns yellow, then turns red, then becomes highlighted or blinking, then you know that you are approaching death and requiring healing or rest. This patch makes no change to gameplay but can save your life!
HPmon is also popular because nethack.alt.org had it (NAO switched on 2012-05-19 to statuscolors). (The version there sometimes fails to color the HP meter; this might be a bug after resolving the conflict with the dumplog patch.) GruntHack has an expanded version of it.
Compare with the statuscolors patch, which extends HPmon to color several parts of the status line but requires user configuration.