With most motherboards as long as you have all the drivers installed (chipset, onboard devices, etc) and your motherboard supports SATA II than the 3GB/s will automatically "kick in" so to speak.. The chipset drivers are most often what controls things like that..
There is no "flip of a switch" or button to push that "enables" 3GB/s.. like I said in most cases it's the motherboard drivers that take care of that kind of stuff, so just make sure you have all of those installed and that your motherboard supports SATA II and you should be good to go..
It's just like the old IDE HDD days.. you could install an IDE HDD that ran @ 133 but if your mobo only supported IDE devices up to 100 than it would automatically clock the drive down to the maximum supported speed..
Same with RAM in most cases.. (Not all cases though with RAM, it's a bit more picky that way sometimes..)
As for why someone would want 3GB/s.. well why not!?
It's twice as fast as SATA I... As for noticing a difference, well that all depends on what your doing.. if your doing something like heavy video editing than yes you probably would notice a difference.. but for just everyday use (web surfing, emails, chat, burning music cd's, etc) than no you won't notice a difference at all.. Plus I don't think we're to the point where the full speed of SATA II is fully utilized yet anyway... so even IF a difference is noticed, it would only be a marginal one at best.... (unless of course your using a Velociraptor HDD, but than your getting into much faster RPM's and more cache to work with so their kind of an exception..)