The Phantom Help Mavic Pro battery cover protects the Mavic Pro battery from a short circuit. It’s also handy when storing batteries in a loose bag or traveling with them in locations where they could potentially get dusty/dirty.
When flying in the US, the FAA requires the battery terminals to be covered (see more details here). The battery cover securely connects to the battery and is slim enough to fit into most battery compartments while attached to the battery.