A variac gives ac fans 0-100%+ control without any buzzing or humming from the fan coils.
Depending on your local environment it can be better to run the fan constantly slow/er for a more stable environment.
If your climate doesn't suit it's simple enough to add a relay for temperature and humidity control, that's basically a hybrid controller (zero buzz) that would cost you hundreds of pounds to buy a basic model, the last hybrid controller i bought cost me £250 that was without speed control or humidity control.
They're completely useless for Dc fans.