How do bones provide a structure for muscles to move?

Think of muscles kind of like a slingshot. If there wasn't a sturdy enough frame to withstand the force of the elastic band, then we wouldn't be able to shoot anything.

Without a sturdy frame for our muscles to grab hold of, there wouldn't be enough tension for our muscles to move. Without bones we'd be an awkward pile of skin, muscle & bodily fluids.

Hope that helps!