Can someone please explain why by adding a knob of butter into the pan of water during the steaming of cabbage makes the cabbage taste better. I understand this idea come from Denmark. I've tried it and for some unknown reason works and my friends have spotted the difference during a blind test I did.
Some of the butter ends up on your food. Most people will think the flavor is improved if there's a tiny bit of salt and a tiny bit of butter flavor. We like fat, we like salt, and we like the actual butter flavor, though it's more subtle.
In case you were wondering why the butter ends up on the food... The oil melts into the water boiling on the bottom. It'll be trying to sit on top, but with the water bubbling and boiling things mix a bit. Either way, little droplets are definitely going to get thrown around. A bit of butter, or even most of it, is going to end up on your food.
You can easily prove this to yourself - the residue on the sides of the pot will also be greasy, and if you boil away all the water after you remove your cabbage, there won't be nearly as much melted butter as you started with. You can also see this if you're cooking things in just oil (with a bit of water coming off the food). As things splatter, you get a lot of oil droplets flying everywhere, which I'm sure you've noticed when you're cleaning in the kitchen. Actually mixing oil and water just makes this even more pronounced.
I'm not sure how you do your steaming, but if you're taking a shortcut, skipping the steamer basket, and just putting a bit of water in the bottom of a pot or pan, even more (probably most) of the butter will end up on the food.
Well, adding things that taste good to a dish makes the dish taste good, and butter tastes good. If you added something that tasted bad to the cabbage it would taste worse. It's the essence of good cooking.
If you are asking why butter in particular is good, butter is an emulsion of fat, milk solids, and optionally salt. The milk solids have flavorings, the salt never does any harm with cabbage, and the fat adds not only flavoring but mouth feel. Butter is semi-solid at room temperature but melts not far above it. Butter fat keeps a nice, viscous coating property that oils don't have. If you put in sunflower oil instead and did another blind taste test your guests would probably tell the difference.
Animal fats generally have this quality as most become solid at room temperature. If you fried some bacon and added it you would have a similar experience to the butter.
EDIT: I see from the comments that the butter was added to the water steaming. The butter and solids will end up being vaporized in a mechanical process as the water is boiled and evaporating.