Bicycles have one effective brake (in the front) and a backup "slow-down device" in the rear.
What if you suddenly brake hard, wouldn't the momentum flip you over the front?
Yes, of course.
If you never use the front brake out of fear to flip over the bars, then you won't be trained to use it properly. Then when a driver piles on the brakes right in front of you because something happened further ahead, and their car screeches to a halt... your hand will squeeze the rear brake because that's what you trained your reflexes to do by always using the rear brake.
About a second later, you realize you're not slowing down nearly enough... and your rear wheel is also skidding. So you try the front brake. But you've already wasted precious braking distance, and you never use the front brake, so you can't automatically apply the proper pressure by instinct. So you either don't squeeze enough and crash into the stopped car, or panic and really squeeze it, then flip over the bars and crash face first into the rear window of a stopped car.
However, if you use the front brake most of the time, especially in situations when there is no emergency and the rear brake would be fine, then you can really train your reflexes to use it properly. Then, when you actually need to brake hard to avoid crashing, you will be able to apply the maximum braking power that still keeps the rear wheel on the ground almost immediately.
It's one of these paradoxes: if you're too afraid to flip over the bars and don't learn to use the front brake, you'll eventually flip over the bars anyway.
Do not practice on a slippery surface, though.