Oleum is an important intermediate in the manufacture of sulfuric acid due to its high enthalpy of hydration. When SO is added to water, rather than dissolving, it tends to form a fine mist of sulfuric acid, which is difficult to manage. However, SO added to concentrated sulfuric acid readily dissolves, forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid.
Typically, above concentrations of 98.3%, sulfuric acid will undergo a spontaneous decomposition into sulfur trioxide and water
This means that sulfuric acid above said concentration will readily degenerate until it reaches 98.3%; this is impractical in some applications such as synthesis where anhydrous conditions are preferred (like alcohol eliminations). The addition of sulfur trioxide allows the concentration to be increased by means of Le Chatelier's principle.