Beau Brummell didn’t invent the suit but he sure as hell made it cool. He looked at the frock coats, knee breeches, and powdered wigs and said, “Nope”. The 18th century saw suits become functional, wearable, and fitted. Brummel, a friend to royalty, was in the process of developing new military uniforms for one of the English regiments and decided that fit, fabric, and cut should be front and center and men should show off their curves (so to speak).
Sure, suits existed before Brummel, but they really sucked. They were burdensome, uncomfortable, and boring. Think of early suits as turkey sandwiches. Without mustard or mayo, you’re basically eating a slice of dry bird. Brummel added the condiments.