If you overfill your cars engine with oil you risk damaging the engine.
When the oil level in the oil pan is too high the oil cannot circulate properly and lube the engine and it's parts like it needs and you will eventually have a locked up engine.
If you overfill your engine oil by just a tad it's okay and not likely to do damage but if you overfill it too much you should always drain the excess oil back out to avoid engine damage.