Are you speaking about their origin? Because Phantomhive and Hillboy explained that.
If you're speaking in terms of plot relevance, they existed within the story as a means for Vanitas and Master Xehanort to test Ventus and make him strong enough to create the X-Blade. They were also used to draw TAV away from Master Eraqus and isolate each of them sequentially, as the appearance of the Unversed upset the balance and peace of the Worlds (leading Terra to go investigate, which influenced Ven to follow Terra, resulting in Aqua chasing after Ven to bring him home). It was one grand diabolical plot.
What I wonder is why there are no allied beings (until ow, finally with da good Dream Eater Spirits).
If Vanitas had evil Unversed (which is a really bad name) why not Ventus with an opposite?
Heartless are beings of darkness; generally they ally with (or respond to) whomever has the strongest source of darkness in their Heart. Sora couldn't command them because he has very little darkness in his Heart.
Nobodies appear to have a pretty fixed hierarchical instinct to them; the stronger, more powerful Nobodies command the lesser ranks and they won't really respond to anyone else.
Unversed were controlled solely by Vanitas. Ventus didn't create Unversed of a positive sort because his Heart was far too weak to manage such a feat, and because there was no instigation. Vanitas's feelings of negativity as a being of pure darkness literally overflowed until they manifested into a physical form. As we've seen with beings of pure light, the equivalent is not true with feelings of positivity, which aren't as difficult to carry as feelings of pure negativity. This is because beings of pure light can apparently feel anger and sorrow, but feelings of pure darkness wouldn't appear capable of the reverse: whereas Ventus still felt natural emotion despite his Heart being composed only of Light, I think Vanitas's Heart having no Light to balance out his Darkness actually prevented him from feeling true happiness or joy. Also, Vanitas was of an abnormal existence (so his powers were unpredictable by extension), whereas Ventus was organic in his (so his powers wouldn't really be anything out of the ordinary).