Grammar at its best promotes clear communication but more often is used as a social tool of control and exclusion. When you are already talking to people within your in-group, that impulse isn’t necessary.
On some level. Thing is it is visible and everybody knows what the standards are, social mobility is possible under the sign of grammar.
If the game is wearing a $20k watch or understanding the covert signs of status that you might find in a particular community, that's something different.
Everyone uses grammar. What you're describing is elitism and elevating one particular dialect above all others.