What is purpose of fullers on blades?

[motan]

This is not a very educated answer, just logical guesses.
As some of you mentioned before, weight distribution and especially reducing weight while keeping stiffness and size intact make lots of sense. Steel beams and construction elements in modern use are never a solid bar, but have I, H or hollow pipe shape for the same reasons.
The second possible reason is related to the "blood groove". A straight stab may make a dagger or sword hard to pull out because of the suction forces. Fullers could make a weapon easier to extract. In a way, like some modern chef knives have depressions on the blade to prevent vegetables from sticking to the blade by suction.
But like many other features of weapons, function has become fashion and individual cases would be hard to explain because the functional aspect may no longer be relevant. For example, the central ridge construction has been carried over from bronze to steel weapons, where, at least in some cases, are not necessary anymore.