It's just reps and glorified practice. But it can also be good for developing talent and coaching especially with quarterbacks and receivers. it kinda started in the mid to late 2000s when instead of a school playing against just one other team in a head to head 7on7 practice session, they would invite 3 schools to make a day of it and over the years people started making tournament style meet ups. Eastern Guilford has had one with the Holt brothers for many years now. I know the one at Elon every year also has a lineman camp at the same time as the 7on7 round robin style.
Nowadays youve got so many national and regional tournies with amazing facilities, of course people are going to utilize that. 🙂