From Nasim Mansurov, the author and founder of Photography Life.
Teleconverters do not affect optical characteristics of lenses – they only magnify the center portion of the frame. This means that if one were to use a telephoto lens with a short minimum focus distance, it could be used as an excellent option for extreme close-up / macro photography as well. For example, the Nikon 300mm f/4E PF ED has an impressive minimum focus distance of 1.4 meters. Although the TC-20E III (2.0x) teleconverter would degrade its sharpness significantly, the 1.4x and 1.7x teleconverters would extend its reach significantly all the way to 510mm. At such close focusing distance, the lens will have its reproduction ratio increased by the same multiplication factor of the teleconverter lens, so it will go from 0.24x to 0.41x with the 1.7x teleconverter. A nice option for occasional macro work for sure! Similarly, when macro lenses are coupled with teleconverters, their reproduction ratio gets increased as well, allowing for even closer than 1:1 magnification. However, if one desires to decrease the minimum focus distance of a lens, it is only possible to achieve that with the help of extensions tubes, close-up lenses and lens reversal tricks.
For longer focal lengths, it is often advantageous to combine extension tubes and teleconverters. Placing the tubes between the lens and converter gives the highest magnification ratio, but often at the expense of image quality and/or autofocus ability. Putting the tubes between the camera and the TC mounted directly behind the lens will yield slightly less magnification but is sometimes more usable from image quality and AF considerations. For more, please see this article.