cro probes checking
A square-wave is easier to generate with stable amplitude and frequency.
It is also easier to check the scope with a square-wave signal than it is with a sinusoidal one. Try measuring the period of a sinewave, compared to a square-wave. The sharp edges of the square-wave signal makes things a lot easier. A similar thing goes for the amplitude, too.
Moreover, checking the probe compensation is very easy with a square-wave, since you can easily see if the flat top and bottom are distorted. With a sine-wave you would see the amplitude of the signal change as you adjust the probe, but how do you know when the amplitude is right and the probe correctly compensated?