Differential amplifier IC vs op amp with resistors to make it amplify a differential signal
An instrumentation amplifier IC has better performance compared to a simple opamp - better gain, bandwidth, common mode rejection, lower noise at high gains, etc.  But the real advantage is those resistors.  Check the datasheet, but they usually are laser trimmed for much better ratio tracking than is possible with 1%, or even 0.1% tolerance external resistors.  This is where the superior common mode performance comes from, and that is arguably the single most important parameter when dealing with a bridge signal source.

Also, the internal structure is different from that of a single opamp operating as a difference amplifier.  The gain is set with a single external resistor that does not upset the ratio balances of the other internal resistors.  If you need adjustable gain for calibration purposes, this is a huge advantage.