If you are just beginning packing or an experienced packer it is very important to have a packing list.
I have different packing lists for when I hunt from a trailhead, pack in for 4-5 days or when I am in the back country for 2-3 weeks.
With computers it is very easy to make different packing lists and then modify these lists based on your experiences when you have different packing trips.
One very experienced packer and hunter I know forgot to pack his ammo and only determined his problem after he had packed in and began preparing for the hunt the next day.
A very bad experience that ruined the first day of hunting. You can be assured my friend learned his lesson and now uses a packing list.