Why calculate an estimated displacement?

Frank Fox had suggested to me that displacement would be a better comparison of size between sailing warships than burden, as burden is essentially just a "gross tonnage" measure, having to do with some artificial calculations about ship volume (which is what gross tonnage is all about). The English burden calculation is rather nasty, in that it does not even use a real depth in hold measurement, but uses half the beam. The keel length even was abstracted away from the real keel length in the latter 17th Century, when the English started using a fraction of the length on the gun deck.