Re: Maybe I'm ignorant, but?
It allows a SAS controller to rapidly squirt a chunk of data and a queue of commands to a disc's buffer at 12Gbit/sec, then while the disc is (relatively) slowly chundering away doing as it's told, the controller can do the same with the other 7 discs it's hooked up to (for example.) Once the first disk has completed its operations, the controller can then come back to it to retrieve data from its buffer or issue new commands, all while the other disks are getting on with their own tasks.
In other words, if you have x discs which can only be communicated with one at a time on a bus, each of which can read/write at y GBit/sec, you theoretically want a bus that can cope with x times y Gbit/sec. to maximise disk I/O throughput.
SCSI and SAS standards were designed with multiple discs and multiuser servers in mind, so are generally better at this sort of thing than SATA, though I believe the difference isn't as great it used to be as some SATA standards now include stuff like command queueing etc.