This feature allow users (gamers) to surfing the web in an easy way while in the game instead of switching from window to window to access the game and the web browser app. All of this without interfere with your gaming experience.
Examples:
World of Tanks running on Windows 10, with Overwolf's browser inside the game.
Warzone running on Windows 10, with Overwolf's browser inside the game.
Tom Clancy's Rainbow Six Siege running on Windows 10, with Media Player Overlay playing a song.
An unknown game, with Spotify Overlay (Media Player) playing a song.