Why do you marinade steak with onions

It's a regional preference on what they're called.

When you're buying seeds, they're also called "bunching onions", and I grew up calling them "scallions", although I think that scallion specifically don't have a bulb yet formed, while green/spring onions might.

update : A little research suggests that "spring onion" is the preferred term in the UK, AU and Canada, "green onion" in the US south, and "scallion" in the US northeast, however I'm not sure what the exact boundries are as in the US mid-atlantic, I'll see both "green onions" and "scallions" for sale, with "green onions" being larger (scallions with no bulb, maybe 12"/30cm long, while green onions might have a 2"/5cm bulb, and have over 24"/60cm of green top). It's possible that "green onion" might be a polysemous term that varies by region.

Sauteed onions can provide both caramel flavors (from the sugars in the onions) and Maillard reaction compounds, depending on how they are sauteed. Thus onions can supply a range of "umami" flavors for soup which otherwise you need to get through roasting animal bones and other tissue (e.g. brown veal stock). Of course, even beef stocks often add onion as well for extra flavor.

As an extreme example of this, I often prepare a vegetarian French Onion soup using a meat-free broth made entirely from onions, onion skins, and cheese rind. Blind tasters often fail to distinguish it from a store-bought beef stock.

You can get a lot of the same flavors from combinations of other browned vegetables, but onions neatly supply a perfect package of flavor compounds in one inexpensive, long-keeping root vegetable. Why use anything else?