I think in the MIDI specification this wasn't mean so much with pianos in mind, if at all, but rather for synths to have just one more dimension for expression. I would wildly assume that this may come useful with ethnic types of sounds where the pitch may suddenly blip at the end of the note and depending on the velocity with which you release the keys.
Now, to pianos. If you release the keys slowly (and the damper pedal is not pressed at that moment), the dampers would gradually dampen the string which means the tail of the note would be longer and not abruptly cut-off. On real pianos that's sometimes called finger legato where you play the next note while you're still holding the previous key, or while you're releasing it slowly, so that the notes connect smoothly and are almost blended like with a pedal.
On my Yamaha N1X this effect is supported, although it's very subtle and I have to specifically listen to it to notice it. But I guess as with many other things in life and in audio, subtle effect are nevertheless cumulated in our perception and brain to result in a better ultimate satisfaction at the end.
BTW, the Yamaha implementation is not just about the velocity of the released keys. It actually detects the actual position of the keys, hence dampers, depending on the key depth. Or in other words it's a half-damper effect linked to keys too, on a per key basis, not a global one such as the half-pedal effect which affect all the dampers together. Yamaha implementation is one step further since the release velocity assumes a single event that ended: a release. Whereas the Yamaha implementation will support the key being released only slightly to a middle position after which it can be pressed again to restore the natural sustain of the string from that position onwards.
In reality on a well regulated grand, there's not such a discernible effect and the dampers are mostly instantaneous, but yet, the effect exists to a degree. Listen to real acoustic piano recordings and you will hear little buzzes from time to time: these are caused by very slow release of the keys.
