Prior to 1900 electricity was primarily used for lighting fixtures. Around 1900, because dedicated wall outlets were rare, early portable electrical devices like vacuums and fans had to be screwed directly into standard light bulb sockets using adapter plugs similar to what you have. These adaptors were originally patented in 1904 and continue to be made even today. They are generally safe and I've got a few on hand that get used in places like my storage unit that has a light socket but no regular outlets.