Could you explain why we need the step "Wait for A/D conversion to complete" in ADC module? I've read data sheet and some tutorial but I'm still confused about this step.
That is like asking why we have to wait for the bath to fill. A bath takes time to fill and the ADC takes time to convert. If we don't wait until it's finished then it wont give the correct answer.
There are chips that convert much faster but they cost much more money.