Your hard drive is probably the slowest component of your computer, with each improvement hard drives close in the gaps, but they are still a long way to go. SATA 3 is bound to increase the interface speed but to fully realize its potential the innards of a hard drive need to get speedier.
SATA 3 also introduces changes which will improve hard disk performance for multimedia operations, such as "a new Native Command Queuing (NCQ) streaming command to enable isochronous data transfers for bandwidth-hungry audio and video applications". Also improvements have been made in the power management, and the connection mechanism for compact devices with connectors designed for the smaller 7mm optical drives in laptops and a Low Insertion Force connector for 1.8-inch drives.
Another interesting addition is that of a feature which will allow the host to manage the NCQ commands. For those who don't know NCQ or Native Command Queuing is a techology that optimized the way the hard-drive reads and writes data by rearranging requests so that instead of being read in the order that the commands were issued, they are instead executed in order of proximity. One way to understand this is that, when the computer is asked to open 3 files, they will be opened in the order in which they are stored on the hard disk instead of the order in which they were opened. Giving some control of this to the host computer means that different NCQ schemes could be used for different applications.
It seems like just a while back that SATA 2 was introduced, but improvements in hard disk speeds are still much needed and welcome. This change will come as transparently as SATA2 since the new standard will be backward compatible with previous SATA versions, and people probably won't even know and wont need to know when they go SATA 3.