The first reason being the amount of space Google allows you. Unlike other services from Apple and Amazon, which allow a sparse 5 gigabytes-worth, Google grants you space for 20k songs. That’s 20,000 or roughly 60 gigabytes, assuming 3-minute songs at 1-meg a minute (which is pretty poor quality, I know).
A second reason is access to the Android Market for music. While I have seen a disturbing trend that publishing companies do not want to play nice with Google, Google still has access to thousands of great songs and albums. More so, the music you purchase in iTunes, is easily uploaded to Google Music.
A third reason, and a second favorite (next to the library-space) to someone who rides the NYC subways is the streaming capabilities. Anyone familiar with the NYC underground, knows that there is zero Internet / WiFi access underground. However, and to my amazement, while streaming only for 5-minutes before entering the subway, my music continued for the duration of my ride, which was 15-20 minutes. I don’t have a clue how it works, but it does, even when I don’t expect it too (no, the music is not downloaded to my phone). And this may or may not be related to Google Music and may be better attributed to my Samsung Nexus Android phone. But does it matter? After all, I still believe in magic.
So what are the possibilities here? I am hoping for some Google+ integration to be able to share playlists and music with friends. I guess we should just stay tuned…