One word = SPEED!
Now, even though SATA II 3.0Gbps and SATA III 6.0Gbps are rated at thoose levels, a mechanical hard drive will never even come close to that bandwidth.
My SATA II hard drives only get above 160MBps, which is far under the 3.0Gbps, or 300MBps.
SATA III was really developed for Solid State Drives, which get up to 550MBps today. Don't bother spending the extra on a SATA III hard drive.
Note that 3.0Gbps is 3.0 Giga-bits per second, and 300MBps is 300 Mega-Bytes per second. It's not really a 10:1 scale, but close enough.