Pitching clarification - relief

I'm having a brain cramp ... What is the difference in the possible relief usage ... What is the difference between using a guy who is a closer in the sixth or seventh, instead of a guy who is say an R1 or R2? :?:
http://forum-365.strat-o-matic.com/archivesn/
http://forum-365.strat-o-matic.com/archivesn/viewtopic.php?f=12&t=197434