One of the advantages of Hydro Formed aluminium is the ability build complex shapes that only place material where it needs to be. A bike frame is like a truss it bears the least amount of load in the center but bears all the weight on the two ends. Traditional steel round bike tubing eliminates unnecessary material in the center by "butting". With Hydro formed Aluminium not only can you eliminate unnecessary material but you can change the cross sectional shape to better resist the expected forces.
Another characteristic of Aluminium bike frames is they tend to be stiffer than steel frames.