MDM is centrally configuring, securing and updating a fleet of devices. In a signage context the fleet is normally Android players, tablets acting as kiosks and any handheld staff use alongside them.
It does the work nobody wants to do per device: enrolling a new unit into the right configuration automatically, locking it to a single application, pushing security patches, and wiping one that goes missing. On a fleet of any size that is the difference between a repeatable rollout and a hundred bespoke setups.
The caveat is overlap. Signage platforms increasingly include their own device management, and running both produces two systems with opinions about the same device, usually discovered when one reverts a setting the other applied. Decide which owns the configuration before deploying, not after.
Kiosk mode deserves specific attention. Locking a tablet to one app is easy; locking it so a determined visitor cannot reach the browser, the settings or the home screen is not, and an unlocked public tablet is a device on your network that anyone can use.