Consider this quote form another thread:
Charge, like mass, is a property of matter.
It is more complicated than mass because simple experiments show that there are two types or polarities.
All matter possesses mass, but not all matter possesses charge.
Experiments show that uncharged (neutral) matter exerts a force of attraction between two masses and that this force is governed by the inverse square law. We call this gravitational attraction.
Work is therefore done on the mass of any matter moved against this force.
Further experiments show that an additional force exists between charged matter, over and above that exerted by gravity. It is further observed that the direction of this force depends upon the relative polarities of the participating charges. We call this electrostatic attraction or repulsion.
Work is therefore done on the mass of any matter that we move against this force.
It is often stated, rather loosely, that work is done on the charge. This is not so. Work is done on the mass of the matter. So you will find the mass of the charged particle appear in many equations.

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'.