Usually it’s done so people in the back row stay visible and the group fits together more tightly in the frame. In a group shot, the main goal is often to see everyone’s face, so having the front row crouch or bend can reduce blocked faces and make the composition more compact.

It’s also often just habit or a standard “group photo” pose. People may assume they need to lower themselves so others can be seen, even when the photographer hasn’t asked for it or when it isn’t actually necessary. In rushed situations, people tend to do this automatically, or a photographer may direct it because it’s quick and familiar.

For sports teams and similar groups, hands-on-knees can also create a more active, uniform look.

That said, it’s not always the best solution. A more controlled setup—arranging people by height, adjusting rows, or changing camera position—can often produce a cleaner result. So yes, there can be a visual advantage, but just as often it’s a practical or habitual pose rather than a strictly photographic requirement.