The 1.6x multiplication factor only matters if you want to compare/relate it to a full frame sensor. 18 mm on APC-C are 18 mm. 18 mm on full frame are 18 mm. The difference is the field of view and the depth of the focus. For some people it’s easier to relate to a full frame size, probably a historical reason from the 35mm film days. I tend not to overthink it and just get used to it and enjoy the hobby
