What does the launcher do with regard to the operation of the game that cannot be done within the actual game itself? Is it due to a technical limitation or simply there for the convenience of the players? Are there alternatives to such methods of starting the execution of games?
Managing updates, versions, mods, and settings can be done from a launcher instead of the game itself and it’s often easier that way. If you wanted to adjust mods in-game you’d need to restart the game for changes to take effect.
Before Steam became the universal launcher, lots of games had their own. Minecraft isn’t a Steam game so it comes with its own launcher.