If you meant muscular system instead of modular system, then they function by working together.
The skeletal system offers structure & a place for the muscles to grab onto. Without a frame for the muscles to attach and pull on, we could not be able to move.
Imagine if we didn't have bones, we'd be an awkward sack of skin, muscle and bodily fluids & we would probably have to wiggle like worms to move. Thankfully, we don't have to worry about that!
Hope this was helpful!