Spend some time in any bike shop and you'll see the Brake Squeezer. It's a person that, uncontrollably, squeezes the brake lever on every bike they walk by. Even the hydraulic disc brake bikes that don't have a rotor or spacer in the caliper. Sometimes it's not the Brake Squeezer, but an accident. Maybe the lever got pulled while trying to pack a bike for transit or putting it in the back of one's car. Either way, the pads have now advanced, and to push them back out to clear the rotor you'll want this piston spreader.
The tapered edge helps to get the tool between the pads regardless of how far advanced they'd gotten and the total thickness of the tool helps to make sure the pistons are retracted as far as they'll go.
It's also a useful tool to retract the pistons before bleeding brakes or changing pads. Failure to do so can lead to some pretty annoying problems.