Why did we need to change the SI?
More accurate measurement facilitates advances in science and technology, which in turn allows more accurate meaurement. But this improving accuracy needs to be enabled by measurement standards and their definitions.
Continuously improving the definitions of the units ultimately makes it possible to have tighter tolerances 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 the IPK always weighs one kilogram exactly. However, studies of closely similar copies tell us that the mass of the IPK was almost certainly changing… minutely. This implied a tiny but known change in the values of all masses. For mass, and for all units, we needed to rule out this type of problem.
Constants of nature, such as the speed of light, are unchanging over time and space and we can give them exact values. Because of this, these constants provide the most stable and exact way to define all SI base units into the future.