If it's anything like Subaru has had since the 80's, there's another cable from the shift fork to the master cylinder area. When the clutch is in all the way and you're on an incline, you can take your foot off the brake and the car won't roll backwards.

Basically it gives you time to move your right foot to the gas pedal without rolling backwards on a hill.

I have mine adjusted to release the brakes just after
releasing the clutch.

They can be very nice if they're adjusted correctly.