When my husband and I set up home in 1962 we considered ourselves very modern to have a pressure cooker.
Pressure cookers cooked food much more quickly than ordinary boiling would have done, because, being under pressure, the water boiled at a higher temperature.
The most common form of pressure cooker produced this high pressure with a tightly fitting lid, with a small valve opening in it, loosely closed with a weight of some sort. Some of these weights lifted off and were easily lost, but ours was a lever arrangement The valve opened and let out steam only when the requisite pressure had been reached.
As the food cooked in steam not water, it was put on a low rack inside the cooker with a relatively small amount of water. We got used to how much water, as it was important that the cooker should not boil dry.
Pressure cookers were only really useful for the sort of cooking that was traditional done by boiling, e.g. vegetables. Vegetables cooked in this way were said to be particularly nutritious because the goodness did not dissolve out into cooking water. The cooker could of course be used with fat for frying without its lid, ie at normal pressure. So some people partly cooked meat at pressure and then browned the outside in fact afterwards.
In some ways, pressure cookers were dangerous, because if the valve outlet got clogged up, the pressure would rise dangerously high and the whole cooker would explode. Nevertheless, we used ours a great deal. Potatoes of a size that would take half an hour to cook by regular boiling, would cook in only 6 minutes.
With the advent of microwave cookers, pressure cookers became redundant.