• Better interface than Windows Media player
• 100s of cool and edgy skins
• Nice looking graphic equalizer
• Nice music visualizer
• Easy to make playlists
• Tiny looking player which gelled with the early-mid 2000s vibe

And most importantly, it really whips the Llama's ass. TBH, there aren't a lot of serious reasons. It was just slightly better than the default music player. I personally feel the skins played a significant part.