There are two types of VST programs: VST instruments (a.k.a. VSTi) and VST effects. VST instruments produce sounds and VST effects change sounds. A synthesizer is a common example of a VST instrument. A reverb is a common VST effect.
In most forms of electronic dance music, sub bass synthesizer sounds are used to produce huge, club-worthy bass tones. They are responsible for the stomach-shaking rumble in drum n bass, the smooth vibrations in deep house, and the fat boom in hip hop.
Much like a turntablist uses a crossfader to chop up elements of a record into a mix, music software can be used to chop up audio in your song. When used tastefully, this effect makes songs more rhythmic, danceable, and energetic.