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ecu uses maf to tell it how much air the car is getting. air volume and rpm are used to look-up the required fuel amounts on the programmed maps.
if the maf isn't working the car will use the throttle position to determine how much air is getting in. should be pretty accurate, but can't compensate for pressure differences, etc.
if there is an air leak after the maf (ie, the aos system is often a culprit - failing aos causes excessive vacuum in the crankcase which collapses and cracks the oil fill tube which then allows air to be drawn in post-maf but pre-throttle body) then the ecu will get thrown off. not so with the throttle position sensor.
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