The purpose of these resistors is to force an output or input to go to a defined state. This may happen for lots of different reasons. The alternative is to let that point in the circuit float. If it is an input then sometimes it will be a zero and somtimes it will be a one. Clearly an undesireable situation.
Some outputs are called "open collector" or "open drain". In these cases the pullup resistor is essential for establishing the high state. Without them the output will be stuck at zero(low).
Answered by: siva85
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