People like previewing before buying something. If I see a trailer, it doesn't say much about the movie -- it's just a narrator, with a heavy voice, making bold note about the plot of the movie. A lot of people hate this and want to see up in front what the movie is about.

Same thing for the album songs -- no-one guarantees you that you'll like it or not, unless you preview some parts of the song (not just the first 5 seconds).

Piracy on the other hand, allows people to preview parts of the media they want to try out, then decide if they like it, and finally to pay to get the "real deal", meaning the quality and support their favourite artist for making something they liked.