quite simply they are for adding and subracting two signals.
sorry had to say that.
You would use that in in many ways, including making sure and adder circuit works correctly, analyzing noise and noise cancelling in circuit. I know I have used subract (or invertng one channel and adding) more than just adding two signals.
For example if you have 2 analog voltages being fed into an adding amplifier which lead to an ADC you would want to know if it was working correctly, you would put a probe on each input to the adder, and one on the output then add the two inputs, if it was the same as the output you know that it good.
Same for noise cancelling, you put one probe on the noisy signal, and one on the noise cancelling signal and see how well the noise cancelling works.