How to stream music on your phone from laptop?

I want to know how to play music on my mobile phone from laptop via bluetooth or wifi? I have Nokia Lumia 520 Windows phone 8 and HP 2121TU Notebook PC.
The phone specification says it has A2DP bluetooth profile.

Why? when you can upload the music to the phone. This would require you to be near the laptop so why not use a pair of bluetooth headphones to listen to the music from the laptop?