Can be useful in some scenarios.

Such as?

Welp, knowing whether the current state start is a cold one, or is actually a "reboot" from another map, without dirty checks/hacks.
A specific example would be i.e. a server subscribing to some sort of event queue on process start, and not redoing that every time a map changes. Another one is posting/acquiring some data only on server start, and doing something else in other scenarios. An example is telemetry.

You are in luck as this value appears to be exposed via gpGlobals->serverCount, added as game.GetMapChangeCount (serverside only)