On older aircraft one of the more common occurrences is that if one bleed/pack fails, the remaing bleed will be supplying both ac packs and does frequently lead to overloading of the precooler or temp/press controls on the remaining system resulting in a second bleed failure and press'n loss.
As soon as the first failure happens, selecting the affected bleed/pack off will avoid the cascade of overloading the remaining bleed/pack. A single bleed/pack will normally keep the aircraft pressurized.