Adding a trumpet turned the sexteto into a septeto, giving it more projection for bigger venues and open-air dances. The trumpet also added space for improvisation, call-and-response with the singer, and a more “urban” flavor. Example: Septeto Nacional (Ignacio Piñeiro).
In the late 1920s many sextetos became septetos. The Septeto Nacional of Ignacio Piñeiro, founded in 1927, was one of the leading groups of the time and still exists today.
The trumpet adds recognisable accents and short lines between the vocals. That makes the music livelier than sexteto son, but the groove stays light and easy to follow.