Tip-down carry means the knife rides with the blade tip pointing toward the bottom of your pocket. It is the traditional orientation, and it is the default on many older designs and on knives without a strong detent.
If the blade should ever start to open in the pocket, tip-down points the edge into the pocket rather than up toward the opening and your hand. On knives with weaker blade retention, that is a genuine safety margin.
It is also simply what many people grew up with, and habit matters when you are drawing a tool without looking.
The draw is slower. The knife comes out of the pocket in the wrong orientation and has to be rotated in the hand before you can reach the opener, which costs a beat and takes practice to do smoothly.
Flippers do not work well tip-down, so knives designed around a flipper tab almost always ship tip-up.