I always wonder about these two systems. I thought I'd ask here.

What are the reasons behind the existence of the two different systems (weight and piece)? Why do we have two different systems? Does this mean carriers flying to and from the U.S./Canada/other piece sytem countries carry more bags from pax than cargo? Who determines what country uses what system?

I know it's nice to be able to fly, for example from the U.S. to Europe, with the two 50lb bag allowance (in Y in this case), but it would be expensive to fly within Europe on a separate ticket with that much baggage since they would charge you for the excess baggage, unless you have a way to leave your stuff somewhere.