Measuring systems are almost always arbitrary - even the orderly metric system is chopped up into random little pieces we call grams, kilograms, and tons. With the increased need for precision, it's necessary that everyone use the same random unit of mass for a kilogram. After a Nobel winning discovery, and decades of tireless research, scientists have managed to pin down a kilogram and an ampere.

In order to get everyone on the same page, mass-wise, people have to be able to use measuring units that are the same wherever they go. They also need to be able to use units that won't degrade over time. This is a tough thing to do, and so far people haven't managed it with a kilogram. The standard kilogram, used by every company and institution serious about measuring things, is a lump of platinum-iridium. That lump, although it's being carefully maintained, is vulnerable to damage, loss, or general degradation over time. In order to measure a kilogram again and again, no matter what, it has to be tied to universal constants.

A Plank mass is a unit of mass defined by Planck's constant, h, the speed of light, and the gravitational constant. Two of those are already universal constants, but one, Planck's constant, is tied to another constant, e. This is the charge of an electron. The charge of an electron could be measured in amperes - unfortunately we don't have a universal constant that can help us define the ampere.