Most demos over the internet can't give you a real appreciation of the actual sound. Capture, video codec for editing transfer, rendering, video compression for youtube, etc. I love the people who purchase a mic or preamp only after listening to internet audio.
The theory is simple - a compressor squashes the difference between the softest and loudest parts of a track, so that the sound is fuller. On screen, a compressed track looks more like a ruler than a bouncing recording - which is exactly how it sounds. As pointed out above, it can keep the level high enough so a supporting sound doesn't fluctuate and disappear under the lead, while a lead will float steadily above all the backing sounds. And for that delicate balancing work you need to hear the details, not something that has been squashed after the artist finished the piece.