Does it matter if I sauté onions for high liquid foods?
It makes a big difference. The heat you can impart to anything by boiling is limited to 100°C (212°F). This is too low for some of the flavor magic to happen, like maillard reactions, which start about 140°C. A pan's temperature can get much higher than boiling water, which is why you saute them first. If you try to make onion soup without sauteing them first they'll soften, but they'll never lose that raw onion flavor, and you won't get the sweetness or depth of flavor from the maillard reactions.