First of all, it's convenient. Typing a single parenthesis is faster than entering a combination of three keys :-). This time can be spent more productively. For example, to type ))).
This symbol is not equivalent to the classic smiley. By using it, we show friendliness; it's more of a polite smile than a demonstration that we find something funny. Agree, "well, you're an idiot)" sounds much friendlier than the rude "well, you're an idiot." And you could get into trouble for "well, you're an idiot."
We, Russians, are too tough to use emojis like the rest of the world. Stern, concise parentheses are our choice! That is, we wanted to say, this is our choice). So know that, America).
The number of parentheses can convey subtle nuances of meaning.
")" - politeness.
"))" - the interlocutor finds your phrase funny.
")))" - you said something extremely funny; consider a career as a comedian.
"))))))))))))))" - such a well-mannered person would not allow themselves to do. Perhaps your interlocutor is 8 years old. Or he went to the kitchen for a beer, and a cat jumped on the keyboard.