I hunt using a climber stand, and I might use several trees on a property during the hunting season. Marking tacks will reflect the light of a flashlight beam, which makes them much easier to see than flagging tape. Tacks placed on either side of a tree make it easier to find your way into or out of an area when it’s dark.
You can also use them to mark the base of a tree you want to sit in, and the direction you want to be facing. Knowledge of the property or a GPS will help you find the right general area, and the tacks will help you find the exact tree. That way I don’t give away my secret spot on public land. Please make sure this is legal in the area where you hunt, and remove all the marking tacks at the end of the season.
The box containing the tacks when purchased often breaks or becomes difficult to close, resulting in them spilling into my pack. To solve this problem, I put them into an old pill bottle, as shown.