Ultimately it comes down to Humidity control at a given temperature - which in turn leads to more even cooking, browning and improved moisture retention. In some situations also reduced cook times.
There are two types of steam cooking - Steam (Sous Vide) which is under 100°C and Steam Assist which is over 100°C
So with steam assist if you want to Roast a Chicken, you ultimately get perfectly cooked juicy white meat but with perfectly browned skin.
Tray of cup cakes? Fluffy and moist inside, perfectly browned tops.
Bread? Doughy artisanal sourdough with a great crispy crust
Sous vide / full steam will let you steam meats and veggies at controlled low temperatures.
Veggies generally retain more colour and vitamins and are tender throughout.
Sousvide a steak you''ll get perfect internal temps giving uniform pink colouring, that only needs quickly searing on either side to taste.