It's mainly for older amps with the paper/oil caps. A sudden voltage surge can cause older caps to short out-which can be very bad. I would worry mainly about the tube gear but if you buy a variac, use it only any solid state gear too.
By very slowly bringing the voltage up, it will allow the caps to "reform" internally and avoid costly shorts or having to replace the caps.
By very slowly bringing the voltage up, it will allow the caps to "reform" internally and avoid costly shorts or having to replace the caps.