dvi = digital video interface= HDMI without sound
D-sub(15pin) is analgoue video (usually some are digital)
In general it is a cable with a new shape plug for video
Is very normal for 新mon to have DVI input.
The difference on video quality is not as much between your normal D-sub(15pin) VGA cable. For most normal usage on a 1-2000HKD mon.
Depend on what type of display card you are using, you may or may not have a socket for this cable to plug. But you can always use your old VGA cable
Also there is a VGAto DVI adapter costing 15-20 HKD to change the head to the right shape, if you want to use the new DVI cable
If you really want you can always get a new display card with DVI socket to use with this cable. But it will cost you at least 5-600HKD with no real improvement in display quality