Over time, concrete slabs can sink into wet or unstable ground. Once soil compresses or erodes away, the concrete will sink into the ground, creating an unstable surface. It can be raised back up with mud or more concrete- mudjacking- but those repairs don't last as long, take a long time, and are majorly messy!
Raising concrete with polyurethane foam is a significantly faster repair- done in minutes! There's no mess, and it's often significantly cheaper than traditional mudjacking.