Many reasons for it being a milestone:-
- some airlines will give you a payrise on attaining your ATPL.
- in order to act as cruise relief pilot typically on a longhaul aircraft you will need a minimum of an ATPL plus have passed the prerequisite sim, ground school and line training. Additionally depending on your companys OPS Manual you will no doubt need a certain level of hours on type and maybe sectors so its unlikely that a fresh ATPL will be set loose as acting commander whilst the PIC is in the bunk asleep.
- in a smaller outfit holding an ATPL will mean that you can now enter into the command training/checking process. Typically smaller piston or turbo prop outfits. In an airline expect many years of apprenticeship ahead of you.
- and you might even get another stripe for your epaulettes if that floats your boat.