Why did we need to change the SI?
More accurate measurement facilitates advances in science and technology, which in turn allows more accurate measurement. But this ever-improving accuracy needs to be enabled by stable measurement standards underpinned by agreed definitions of measurement units.
Continuously improving the definitions of the units ultimately makes it possible to have tighter tolerances, better efficiency and less waste. For example, gears will fit together better and therefore function more efficiently and manufacturing will be able to rely on the dimensions of parts to fit together.
The kilogram was the last SI base unit to be defined in terms of an artefact: the International Prototype of the Kilogram (IPK). By definition, prior to 20 May 2019, the IPK always weighed one kilogram exactly. However, studies of closely similar copies told us that the mass of the IPK was almost certainly changing… minutely. This implied a tiny but unknown change in the values of all masses. For the kilogram, and for all units, we needed to rule out this type of problem, caused by relying on physical artefacts as standards.
Constants of nature, such as the speed of light, are unchanging over time and space. Because of this, these constants provide the most stable and exact way to define SI base units into the future. If we can assign them exact numerical values (based on our best measurements at the time) this in turn defines the units which we use to express these constants.