Watch the Horn Lines 6-7 online guitar lesson by Matthieu Brandt from Jump Blues
Playing triads on the top 3 strings is a useful technique, when you want to create a horn like arrangement.
Because the notes are relatively high, you can make yourself audible without being in the way of a singer or the bass player. It also leaves enough space for another guitarist or piano / organ player.
This groove is often played at a higher speed and is also used in Rockabilly.