The OBC hack allows you to see additional information on your dashboard. Wayne wrote a good article on this topic.
See:
Pelican Technical Article: Boxster On Board Computer (OBC) Upgrade / Boxster Turn Signal Switch Replacement - 986 Boxster (1997-04) -
The first paragraph lists the added display features
When I use the durametric, it tells me the OBC is enabled, but I don't see the outside temperature on the display. It may only appear after installing the 4 stalk switch or the poor man's hack. I haven't done either.