A Virtual MIDI Keyboard is a computer program that you install which turns your computer controls into MIDI output. In essence you can use a Virtual MIDI Keyboard to control programs like production software with a piano like layout used right from your computer keyboard.
You can see in the example image here that this software by Granucon allows you to turn your Windows Keyboard into a piano controller with several knobs and adjustments.
Who Would Use a Virtual MIDI Keyboard?
Anyone who is either on a budget or is producing music/has a need for MIDI control while they are on the go. It is great to be able to use your keyboard as virtual piano keys anywhere you have your laptop.
There are pros and cons to a setup like this. The obvious con is that you have to manually control the knobs or sliders with your mouse. This is cumbersome and usually stops your workflow. It is nice however to be able to play your piano keys without actually needing to lug a MIDI controller around.