What is the use of a -5V supply rail on a bench PSU?
As well as the +/- 12V you've mentioned for op-amps many will run from +/-5V as well, sometimes that can be handy because you can use something like a TPS60400 charge pump to get -5V from a 5V supply. You might like to include it for that reason so you can test your analog section before you've made the final power supply.

It might also be a useful test input for bipolar ADCs and other analog stages that accept a negative input voltage, for example you might want a negative input that is not right on the supply rail with a +/- 12V circuit (although for most of those cases you could also use a voltage divider).

I suspect it's not the sort of thing you'd use often but for the sake of a binding post it's probably worth including in case you ever need it.