Why would there be a need to litter the woods with ribbons if it is light enough to see them?

Not littering the woods. Marking the trail. When it's still pretty dark, the white flagging stands out. Plus as it flutters in the breeze, it's easier to spot against a dark woods.

It is littering the woods and in my state you can be charged fir it on public lands. Hard to believe a hunter could not find their way to their own stand if it is light outside, not to mention other people finding your stand.

I used to use tacks but don't anymore. The reason I don't is that other hunters see them and either remove them or find your stand and do things to ruin it. I always lock my stand but I've had tree ladders and lifelines stolen from my stands. I've also had slobs take a dump right at my tree! I even had a guy in my stand once when I was going in for an afternoon hunt.
Too many people today have no respect for others and some are just plain unethical.
If I hang my stand in a new location, rather than using tacks I reposition large branches on the ground that point to the direction I need to go. I'll also break a limb and let it hang. In the early fall when leaves are still green the broken limb will have brown leaves.

So how many noticed someone pulled up a 16 year old thread so they could advertise for free?

It’s another measure of safety to ensure a safe hunt. Nothing wrong with that.

If they are used on property that isn't yours, they are indeed litter, so are ribbons and other junk that people use to find their way to their stand in the dark. There are things called gps's that will allow those with poor woodsmanship to find their stand in the dark instead of littering the land.