Your post is titled "Contraction Joints" but your question says "Expansion Joint." A contraction joint is just a planned crack location. An EJ has a gap.
It's important to keep rocks and other material out of an expansion joint. Otherwise, if something gets in the joint in the contracted state, it may keep the joint from easily moving as the concrete expands,causing unplanned stresses. Joint material is a good way to keep that from happening, though there are others.