All oils have seal conditioners in them, but High Mileage oils have a bit more. The idea is if you have an old seal that is a little shrunk and a bit brittle, the higher dose will help them expand a little and become a bit more flexible. This can help control a small leak from a borderline seal, if the seal is torn it will not help, if the seal is good it should not hurt. They would test for that.
But if a seal has "dried out" over time, adding seal conditioner can help. Once the seal has been treated, it should be in a better condition even if the oil is changed to a regular oil. The High Mileage oil will leave it better off. The regular oil will not pull the rug out and turn it to dust.
But all this depends on the polymers used in the seals. I've heard some modern cars use better polymers, that should last longer but are also unaffected by the High Mileage seal conditioners, which do nothing to them.
Some (but not all) High Mileage oils are also a little thicker to help control burning and leaking.
I hear Valvoline MaxLife SynBlend (semi-synthetic) is a excellent High Mileage oil. I would happily use that as a regular oil in my car instead of Valvoline DuraBlend if I got a good deal on some MaxLife. Even though I don't need a High Mileage oil.