Streaming lets you listen to your favourite tracks and albums instantly. Its main boon is you get instant access to huge catalogues of music, without having hundreds of CDs cluttering up your house or thousands of MP3s cluttering up the memory of your devices.
These services are legal and above board, but on free services the music is often interrupted every now and then with an advert you can't skip. It can also be difficult to find the exact songs you want to hear, so free streaming services aren't for everyone.