Kick-Out / Diverter Flashing
Not installing diverter flashing can allow water to run down the wall behind the gutter. Because there is a sidewall transition at these areas, there are openings in the wall where water can find it’s way in behind the siding, which can (and often does) lead to damage of building materials (framing, insulation, siding, etc.). This could also induce unseen mold growth.
Here is the link to a video provided by James H. Hardie Company, a well-known national siding company, that shows how kickout flashing is suggested to be installed in accordance with the installation standards for their siding products.