I wouldn't use this product on a marker, but on a fill station this is a great product. No need to worry about checking an o ring before trying to fill. Just screw the tank in and fill. If it's not your tank, why should you have to replace the o ring? It's also easier than trying to fill the tank, and having it leak and freeze your hand.
If you own a fill station, you should buy this. Otherwise it's not necessary.