Consider a boring old LM741 op-amp, with an "offset voltage" of 5mV. That's the voltage you'd have to apply between the + and - terminals to get an output of 0V (ideally, offset voltage is 0mV).
Now configure the op-amp with a gain of 100. The gain not only multiplies the input signal, it multiplies the offset voltage. So your output voltage is generally as expected, except it can be off by as much as +/-0.5V (100 * 5mV). Probably not what you wanted.
Now look at the OP177. Its offset voltage is 25uV, or 0.0025V. Same gain-of-100 circuit would produce an error of only 2.5mV in a signal that has been amplified by a factor of 100. Precision!
The noise behavior is another feature, which is important for dealing with very low level signals (e.g., ECG's).
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