In Rick Atkinson Book "Crusade, the untold story of Persian Gulf War" (90-91) he states in the book that Rangers were operating with Delta to patrol inside the "Scud Box" looking for lunchers can could used agianist Israel, they were attack is possible or to call in airstrikes. He also states the Rangers brought in due to storage of Delta Personel.
Durring Operation Eagle Claw in 1980 the Ranger were used to secure the Desert One location and were used to secure an airfield to permit C-141 transports to land in advance of the rescue to transport the contingent from Iran under the protection of fighter planes, after Delta has stormed the embassy.
http://en.wikipedia.org/wiki/Operation_Eagle_Claw
I could see the Rangers being used a replacments for some SF troops already in AOR, not to Delta unless is present in the AOR, and I though cannon had guarding the JCS, which would make sense to best troops aviable as strategic reserve to used to fill holes in other formations.