What makes steel-cut oats different?

They're coarser, so they have a better texture when cooked. Rolled oats are mushy and bland, even if you get the non-instant variety. Groats are a pain in the butt to cook: like millet, they take forever.

Steel cut oats are a nice compromise (technically steel cut groats). Get some yummy groaty flavor, but the prep time is much lower (a mere half hour, instead of a week or two for whole groats).

Specifically, steel cut oats look like little pellets, whereas rolled oats have the familiar flat shape from going through a steel roller. The rolled ones expose more surface area and therefore cook faster, but produce a different final texture. I do like the steel-cut ones better, but they take upwards of half an hour to cook. You can make a large batch and reheat it for 2-3 days if you like. I think some folks also use a pressure cooker to speed them up.

Steel cut oats and rolled oats are the same initial product, and after processing they basically have the same nutritional profile. Only a low heat is used, and nutrients don't magically fly away

Both are steamed. This is done for shelf life. It quite hard to buy fresh, non steamed oats (steel cut or rolled)

Everything else is personal preference, taste, texture etc

Rolled oats cook quicker (and therefore use less energy :-) ), and can be used in many baking recipes