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?
It usually makes it easier to centralize authentication. But more importantly (from the company's perspective), it gives the game producer a chance to upsell other products and micro-transaction content.
I don't think I've seen games do that, not even to advertise other games from the same publisher/developer. But then, the only five games I've played with a launcher are three MMOs, Genshin, and Honkai Star Rail.
Minecraft is special. The launcher sets up a bunch of Java stuff and other game configurations. And it downloads updates and handles authentication. Technically you don't need it, but it makes it a little easier for the developers and the users. There are also third party launchers with additional functionality like downloading mods.
Other games often pretend they need that stuff when in reality they just want some more tracking that is not in the game itself.
Technically you don’t need it, but it makes it a little easier for the developers and the users.
For that matter, if you poke around in some games' files, you can find the actual game.exe and launch it directly from there, bypassing the launcher. You just bypass the authentication and compatibility checking as well.
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.
The only game I can think of that doesn't actually require a restart is Mechwarrior 5: Mercenaries. I'm sure there's more, but that's what I've encountered.
To answer your third question about Minecraft specifically, you absolutely can run the Java version of Minecraft independently of the launcher. It's just a Java package. Find it and load it with your Java runtime from the command line and it'll play. Without authentication from the launcher, however, you will not be able to connect to Realms or indeed any multiplayer servers that have authentication enabled.
But for that reason, the Java version specifically of Minecraft is famously easy to pirate, provided you are fine with being limited to only playing locally or on pirate servers that have authentication disabled.
In the bad old days this was in fact the only way to play Minecraft in non-supported platforms (i.e. not Windows).
Allow you to set graphics preferences before launching a game, so you don't end up with a tiny 480p sized window on a 4K screen during an unskippable (and unrepeatable) first-launch cutscene; or the reverse, you only see the 1920x1080 (or smaller) cutout of the center of a 3840x2160 image.
I think the biggest thing is convenience. It's a lot easier for the launcher to do some handshakes and check/update to current patch. Other way is client opening, client closing, patch applied, client opens yet again, that type of deal
specific games like Minecraft I think it's nessicary, other games such as call of duty which insists on having one launcher manage all game installs I don't think is.
Imagine trying to handle all the different minecraft installs from the main screen, you would need to launch Java, load a Minecraft game, load a profile, close the game, open the game with the new profile settings and then hope it launches. This setup is very helpful, but I agree most games it is not helpful at all
They do this and keep their top selling games exclusive to their platforms. Games like GTA have shown just how much money you can make off micro transactions. No company is going to just jump out of that market. They use their top games so they are more liley to get people to just have it installed. That way you install the launcher, put in your credit card, and you are only a click away from handing them some money.
There can be positive benefits though, auto updates, update news, community messaging or content.
But mostly so they can get the hardest part of selling done, a valid payment method input.
I googled a screenshot of the launcher, so this could be from an older version. It looks like they have a link to buy merchandise, digital upgrades, advertising more games by the company that you have not bought yet. Also links to submit submit a but, let's them hopefully have less QA personal. Community link to help the longevity of the game for longer term sales.
Not sure about Minecraft, but sometimes the launcher is simply there to make configuration, startup options, parameters, profile selection, etc. a smoother experience for the user. You can do this kind of stuff without a dedicated launcher, but some software publishers prefer to divide things up this way.
It's a dumb analogy, but I kind of think of it like restaurants. Some restaurants have a full wait staff with a maitre d, some have walk-up counter, some have a kiosk with a screen, etc.
Sometimes updates require a game to be restarted. With a launcher it never needs to REstart, it will be started fresh afterwards anyway.
I've noticed Genshin Impact on Android (which doesn't have a launcher) regularly needs to restart after updates, while the Windows version (which does have a launcher) never requires in-game updates nor restarts.
It's also in general a good place to centralise account management (esp. if parts of the game would need to be reloaded if another account is logged in!)
Lastly, although this is more of a side-effect, but it gives a good place to shows recent news/posts of the company about their game to players, such as when there'll be downtime, in a way that's practically impossible for the players to miss.