Bandgap References - What's the need for a PTAT voltage if we can invert a CTAT?
The voltage from a single device not only varies with temperature but also with the current through it, the specific processing of the device itself as well as non-idealities such as terminal resistances. Using two identical devices at different currents (or current densities) cancels all of those items to give a voltage virtually independent of real-world non-idealities.

Many temperature measuring devices actually use just a single device and sequence it at two different currents at a few hundred Hz, then subtract the two resulting voltages to give the voltage proportional to absolute temperature (PTAT).