Any time I don't remember what notes are being played on a track, I pop this in and watch the keys. So much easier than squinting at the piano roll. Very handy. Long fade times can spell out those chords for you, just start and stop playback on the chord and it will sit there long enough for the slowest person to locate. Love this thing.
There are so many times when I want to scan through VSTs and AUs quickly without the fuss of starting up a DAW.
Much easier to use a Host such as Plogue Bidule or Blue Cat's Patchwork and put the Virtual Keyboard or Virtual Keyboard synth first in the chain. There are lots of synths and sampler instruments that don't feature a piano style interface and you're pretty much stuck without an app like VK.
There is depth to the app, too. There are a number of options that add flexibility and, thus, value to the software.