The manifold sensors are reading what the engine is actually burning. The post cat sensors are reading what the total burn is (engine + catalyst).
Say you go catastrophically rich - a coil pack fails, so you are chucking unburnt fuel into the exhaust. The manifold cat will say "rich" - I've got more O2 than expected in the gas stream. The post cat sensor will be reading what it always reads - but the cats will be getting very hot as the fuel is burning in there rather than the engine.
Simplest way of saying it is the post cat sensor tells you what the overall mixture is doing. You've chucked x Kg of air in and y kg of petrol and that is the right ratio. Tells you nothing about where it is being burnt, or whether cylinders are misfiring etc. It will tell you if an injector has failed.
Pre cat sensor tells you exactly what is happening in the engine, tells you nothing about the cats.