Front brake is where it's most beneficial as the front wheel is what keeps the bike upright. If you lock the front wheel, especially in a turn, the bike falls over almost instantly.

edit: Rear wheel locking gives a bit more time to react.

Personally the main safety benefit is for emergency braking in wet conditions. ABS also allows to use full brakes in the wet without worrying so much about falling over. That's the real benefit, real-world it shortens your stopping distance because of that increased confidence in using the brake.