Sam needs an angel who was around at the creation of Purgatory to tell him what it's made of and how it was built. The most convenient angel is Lucifer -- but Sam's connection to / memory of / trigger for seeing Lucifer got off-loaded into Castiel, who is now missing. If Sam wants to mine Lucifer for information, he's going to have to fix that. He's going to have to break what Castiel fixed.