I had solo piano lessons as a child, but in the old days, pianos were expensive and most teachers could only afford one instrument ;). Nowadays, digital pianos and keyboards are so affordable, it isn’t difficult for a piano teacher to own several of them, so piano lessons don't have to be solo anymore.
Many parents prefer solo piano lessons for their children because they assume the individual attention will produce a better pianist out of their child. This leaves us piano teachers working hard to “sell” group lessons as a good thing. Fortunately for me, I had a long waiting list, and the only way I could reduce it was by offering group lessons.
Nearly everything our children learn, they learn in groups. My daughter's dance and gymnastics classes had about 15 kids at a time, and I paid good money for this non-individual attention (until we both decided that neither dance nor gymnastic was her thing).
The friendships, motivation and fun experienced at school, dance, karate, swimming, soccer etc., can also be experienced in piano groups.
As well as having fun together, my group students are better sight readers, generally enjoy their lessons more, and move through their method books much faster than my solo students. They usually practice more too because no-one wants to be the only student in the group who can't play the music.
Another very important aspect of group lessons is the income. Instead of teaching two students for 30-minutes each during an hour, I now teach 3 or 4 students in that same amount of time, which is a income booster.
So if you’re asking yourself, “Why should I offer group piano lessons?”, my answer to you is, “Why not!”