John Deere D105, Is it a Brake or a Clutch?

Is the left pedal on the mower a brake or clutch? I haven't quite figured it out, it acts as a brake, but I see people push it in when turn the mower on, going into reverse, neutral, etc.. Almost like a clutch. So what actually is it? And does it matter if you push it in when using your mower?
Here is what I'm talking about if you don't understand https://puu.sh/vy7rR/d57fdf6607.png

It is a brake as far as parking brake and for stopping. It is also a clutch for disengaging the belt to shift gears.