Consider this quote form another thread:
This 'definition' is the one that leads to the establishment of a unit of charge.
It also explains why we have introduced the concept of charge.
This is a very important distinction. The experiment came first, the concept followed, not the other way around.
But the definition is mute on the subject of why it is necessary for charge to exist at all it simply accepts that it must be so to explain an observation and gives it a name.
It is not 'proven'. To prove something you must have a hypothesis to prove ie you must have a charge hypothesis before the observation.
I call that a 'given'.