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.