It is a matter of habit. If it is your habit, for example, you tie your shoes in a (modified) granny knot every day, and you go through your whole life having to retie your shoes often. OTOH, you can be in the habit of tying them square, and never have them come undone.
With the lantern, after you tie the knot, if you would like to do the best possible job and have the system work as well as possible, you even out the mantle so it is not all bunched up anywhere. If you tie a granny, it comes loose. If you tie it square, it stays tight.