It’s for the belt tension actually! Like a guitar tuner. It’s just one tool in the process though as it is not just the frequency of the belts that matter. Instead the frequency/resonance helps get the belts into similar tension before doing more adjustment.

Spectroid is great! I use it to tune my 3d printer.

Krita is amazing. The color-fill live layer is fantastic.

Last time I tried it, the liveCD booted fine but the full install had an extreme yellow tint to it.