Being prepared can mean a lot of things. You may have a year's worth of food set aside, gas and a generator, warm sleeping bags and guns and ammo. That's great, as far as it goes.
But what do you do if you pull the trigger on your gun and it doesn't go "Bang!"? Yeah, you do immediate action and remedial action, but still no "Bang!". Well first, you'd better hope you're not in a fight. Second, you'd better have some texts on basic gunsmithing for the guns you own along with some spare parts, because chances are your gun is broke.
Teddy Jacobson has some good advice on spares for a number of pistols. Worth a read.