Some devices do not have any actual output VOLTAGE, such as the LM339 comparator IC. It "looks" like an op-amp, but it is a comparator. The output of any of the 4 comparators is always either grounded or open (high resistance from ground). If no PULL-UP resistor is present, then when the device is "NOT grounded" its output pin will not rise to VCC. A high value resistor is connected from +vcc to the output of the comparator. When the output is UNGROUNDED, you get a +vcc reading. OP-AMPS such as the 741 or Lm324 have actual output voltages and therefore do not need the "pull-up" resistor on the output pin.