Let's say that the grid is running pretty close to its maximum capacity. Something causes a power plant to suddenly trip off line. The "something" might be anything from a serious lightning strike to a geomagnetic storm to a bearing failure and subsequent fire in a generator. When that plant disconnects from the grid, the other plants connected to it have to spin up to meet the demand. If they are all near their maximum capacity, then they cannot handle the extra load. To prevent themselves from overloading and failing, they will disconnect from the grid as well. That only makes the problem worse, and dozens of plants eventually disconnect. That leaves millions of people without power.
The same thing can happen if a big transmission line fails. In 1996, there was a huge blackout in the western United States and Canada because the wires of a major transmission line sagged into some trees and shorted out. When that transmission line failed, its entire load shifted to neighboring transmission lines. They then overloaded and failed, and the overload cascaded through the grid.
In nearly every major blackout, the situation is the same. One piece of the system fails, and then the pieces near it cannot handle the increased load caused by the failure, so they fail. The multiple failures make the problem worse and worse, and a large area ends up in the dark.
One solution to the problem would be to build significant amounts of excess capacity -- extra power plants, extra transmission lines, etc. By having extra capacity, it would be able to pick up the load the moment something else failed. That approach would work, but it would increase our power bills.
At this moment, we have made the choice as a society to save the money and live with the risk of losing power. Until we make a different choice, then, it makes sense to be prepared for those times when electricity is not an option.