Shoeless,
This question goes right along with an issue I had with no oil to the rocker assembly. Please read this thread and I'm sure you'll have a better understanding about the whole top end ie valve/rockers. http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=006285

Basically, pull your valve cover off and make sure you're getting plenty of oil when the engine is running. The plug you mentioned is access to a passage that oils the rockers/rocker assembly. The head bolt located inward from that plug should have a tapered end that allows oil to flow up the "rocker oil tube"(aluminum looking tube connecting both rocker shaft ends). If you're not getting oil (like I was) someone may have installed the wrong head bolt (non-tapered) and you'll have to find it, ONE by ONE re-torqueing to 90-100lbs.

So, in a nut-shell other that it being helpful to identifing oil flow I'm not sure of any-other uses.

Good luck,
j