(1) The carriers are slightly different between SAS and SATA drives. The SAS carrier has a peace of metal that makes the drive fit further back in the carrier because it does not need an interposer. The SATA carrier allows the drive to sit further into the carrier so the interposer will fit.
This link shows the drives and the carriers.
(2) The interposer is required for SATA drives when being used in an MD1000.
The interposer allows for path redundancy within the enclosure. SATA drives have one port for communication where as SAS drives have two. Also without the interposer card I have seen, first hand, incorrect informatuion reported by the OMSA storage management utility.