The camera user manual says exactly that. The reason is if you leave it in the battery compartment although it is electrically
"off" or not switched on the reality is there is electrical activity within the battery since it is a chemical concoction.
Technically all batteries will self discharge to zero which is a problem as when a battery is deeply discharged for rechargebles (to lower voltage too low to operate camera) it may be hard or impossible to charge it via normal means and so render the battery useless so it pays to charge from time to time less in frequency so for Lithium types which is standard supplied for cameras.
The worst thing that can happen is when the battery leaks it's chemicals which is acidic so it will corrode the battery contacts in the battery compartment so much so that even scaping of the gunk off the metal is of no use and the camera is rendered inoperative or
layman's lingo .. not working.