Joints are like an after-school program for cracks in concrete. If we fail to provide a place for cracks to occur safely, under supervision, and in aesthetically pleasing configurations, we as a society will be faced with complete anarchy in our walls and slabs.  This horrifying chaos could lead to unwed, underage cracks begetting more cracks and, possibly even more alarming, cracks on crack.
Sometimes, despite the provision of joints and reinforcing in concrete to resist tension, construction goes horribly wrong. Say the mix is off, or the wrong strength is used by mistake, or perhaps …