it would appear that groove~ requires two types of info in the left inlet: sample number playback position (your 0) AND playback speed. Sending the number zero will do nothing; sending a continuous signal (sig~) provides speed value but also initializes playback:
@Byakko remember that MSP is a signal network and the groove~ object requires a constant supply of 1's so you use sig~ to keep sending 1's over and over to the groove~
At least thats my undertanding of it. Sorry, I'm a noob too, so that's as explained as I can get it lol!