Dedicated Server - GlobalDefaultGameMode and GlobalDefaultServerGameMode

I don’t understand the concept behind GlobalDefaultGameMode and GlobalDefaultServerGameMode

Imagine you have two game modes.
One for the lobby and one for the playing the game itself.

When a player connects to the server he should go to the lobby.
When a player starts a game, he should uses the mode for playing the game.

Why do we have

In the section [/Script/EngineSettings.GameMapsSettings]

server and game keys for map and game mode ?

Assuming on the server no map will be displayed since nothing is displayed.
And for game mode, only the player will be able to interact with the game, not the server.

Therefore, what is the purpose of having a map for the server and a map for the game ?
Is the server using ServerDefaultMap and GlobalDefaultServerGameMode when it starts
and the player GameDefaultMap and GlobalDefaultGameMode ?

Txs for your help,