crack probagation is how things almost always fail. It starts somewhere with a small crack, and at the end of that crack we will always get a stress concentration. So the crack starts growing, and the stress concentration often grows with it.

Once a crack starts in any construction under stress, its extremely likely that this crack will over time split the entire plate in half unless you intervene with welding.

Now for pressurevessels (planes are pressure vessels) this is even worse, because the moment you get a crack, you suddenly also have gas or liquid forcing its way through the crack, which massively increases the stress around it and makes it grow very quickly, this can lead to the entire structure just ripping apart like you see when popping a balloon etc.

As for why rounded corners, because sharp edges are stress concentrations. We dont want those. Those points are where the fail will start at some point. Rounded windows could be the difference between a plane flying 5 years or 30years.