[jmwilson]

Motors, wall warts with line-frequency transformers, or other things that are an inductive load. Magnetic materials designed for 60 Hz may saturate at 50 Hz, though usually things designed for 50 Hz will at least sort of work at 60 Hz.

[bluGill]

Other than clocks almost everything is designed for 50 or 60hz. it doesn't cost much more to design for either frequency, and typically you don't care about the RPM difference that much. By handling both you can just sell your things around the world. A lot of money has been lost because someone shipped something to a country it wasn't designed for. Making a motor that can handle different frequencies and voltages tends to be easier than the logistics of getting the right thing to the right place (most of the world is 100-120V, 200-240V, or 400-480V - and those variations are easy to handle if you design for it).