zero is marked, it just means it doesn't stop there automatically as you are turning back down (if you wanted to start over).
this helps if you don't want to be looking up at the turrets instead of through the scope.
if you only want 8 clicks, and forgot where you were, back to lock and count them out again, for example.
there are limits to how many rotations in either direction.
for long range scopes, you'd have the majority of travel is only in one direction anyway if possible.
for example, my leupold mark 5hd has 29.1 mils (~98 moa) of total travel. but i only have about .6 mils down.