Why would you use a feedback resistor on a logic buffer?

The resistor adds additional hysteresis beyond what the chip provides. With the nature of what the internet is these days quite possibly you may have found a circuit idea that was designed before the time that the a Schmidt Trigger type IC was used in the circuit.

It is quite common for photo detectors to operate with very long rise and fall times. When a non Schmidt Trigger type IC chip buffer is used the slow transition signal at the output of the opto coupler can cause the buffer chip to transition multiple times. In some applications this is not a problem bit in others such as a counting application it can be a big problem.

Looks like it's somebody's attempt to "improve" the Schmitt trigger performance of the device, which already has Schmitt trigger inputs (inputs with hysteresis), as you noted.

Since the switching point (voltage level) for most digital devices are only roughly defined (as opposed to an analog comparator, for instance), this falls into the category of a hack, IMO.