The backstage is where Backstage Brawl
type matches take place.
In general it looks like a corridor, but since WWE 2K22 its much more open-ended.
The backstage of the arena, where wrestling comes to life. Also where some battles happen sometimes. Many times. A lot of times.
Through here you can access The Ring, the GM Office, the parking area, the infirmary, and others.
Nero and Adachi fight here before being sent to the TV World.
Many attacks by Black Battler on wrestlers happen (though in reality they are perpetrated by Agent 47 and Max Payne).
Many riots happen against Kojima and his treatment of wrestlers.
The Backstage Uprising happens, accelerating Rumble-17's start. The backstage is destroyed in the event, but 12 years later is rebuilt in a new Coolsville Arena.
The Mystery Masters investigate backstage for the strange happenings related to Vitalik in the company.