When rods is added to increase the depth of drilling, the loosening of the threads are now performed at the bottom end of the saver sub as opposed to the bottom of the Kelly or Top Drive which is the most valuable part of the drill string. This means that the connection between the top end of the saver sub and kelly / Top Drive is seldom used, and suffers minimal wear and tear, whereas the lower connection is used in almost all cases displacing the most wear and tear from the rotary head connection to the much cheaper Saver Sub which is expendable and reduce costs.