I can't add anything to what Mac said, read his post carefully.

Synths use a 'sample' of the instrument. when you hit a note on the keyboard it triggers a note. That note will play the same everytime (or may a little variation depending on velocity or aftertouch, but not much)

Realtracks are actual recordings of musicians playing parts. You won't get a note triggered to sound exactly the same (like a synth), which adds a lot to the 'realness' of the end product.
Some people have become very used to sampled sounds and prefer them as predictable. Some (like me) prefer the 'real' sound of someone playing the instrument with all its nuances.

I do not work here, but the benefits are still awesome
Make your sound your own!