If you live in the world of digital forensics, you see hard drives of all form factors and ages (along with plenty of dust bunnies). You also likely wipe and re-purpose hard drives quite regularly. You know more acronyms than your typical teen textaholic, and you went out and celebrated with friends when Serial ATA was released, because you were sickened by the old master/slave relationships associated with IDE/PATA drives. So why do we still see jumpers on SATA drives? Rest assured my friends, it has nothing to do with a master/slave relationship.
As you probably know, the answer is...it varies. It may be to control spread spectrum clocking, used to reduce EMI, and/or to force the drive to reduce throughput to that of an "old school" host controller that only supports SATA Revision 1.x (up to 1.5Gb/s). If you're wondering why your wiz-bang, high-speed hardware that supports SATA Revision 3.x (up to 6Gb/s) is slower than molasses, check those jumpers my friend; don't let 'em bring you down.