"Ass to grass" squats will develop the posterior chain and the abdominals very well. The quads also grow, but in proportion (because the deeper the squat, the more the power shifts from quads to hamstrings/glutes). The abs are active the whole time stabilizing the weight. That one exercise is more effective than a dozen other isolated exercises.
It also develops neuromuscular control which most people lack. Being able to drive steady, strong, coordinated impulses from the brain to the muscles is critical for being strong, balanced, and capable of doing "anything". The only way to get that is by doing it - using many different muscle groups in a coordinated effort.
An added bonus of doing exercises like that with relatively high weight is that you even out the weak areas. With squats, most people will find abs to be the weakest link at first (if they're paying attention).
It also develops neuromuscular control which most people lack. Being able to drive steady, strong, coordinated impulses from the brain to the muscles is critical for being strong, balanced, and capable of doing "anything". The only way to get that is by doing it - using many different muscle groups in a coordinated effort.
An added bonus of doing exercises like that with relatively high weight is that you even out the weak areas. With squats, most people will find abs to be the weakest link at first (if they're paying attention).