Besides matching the impedance to null the bias current offset voltage, another reason is to limit currents in the case of an input over-voltage condition.
In the case of overvoltage (input beyond rails) most op amps can tolerate a few mA of input current through their internal rail clamping diodes without damage. A resistor of, e.g., 10 kohm hence allows the input voltage to exceed the rails by some 10s of volts.
Although some op amps do contain some serial input resistance, this is usually kept rather small for noise reasons. As a result, external current limiting is almost always necessary when inputs can leave the range between the supply rails.
