USB Extension Cables and Extenders are used to either A) extend the length of an existing cable, or B) transmit USB signals over longer distances that the standard ever intended.
For most applications, conventional USB 2.0 will only work up to around 5m, with that distance being even shorter for USB 3.0, and especially USB-C 3.1 connections. However unless your cable demands are high (like when suppling power and data to a portable HDD), a USB Extension can be used to add a little more length to your cables. Also remember that USB charging performance diminishes greatly over longer cables.
For really long USB connections, we have special USB Repeater cables, and even USB extenders over CAT5!