I should edit the title to "Multiple Begin gameMode" blocks.

I sometimes see multiple "begin gameMode ... end" blocks in scripts. What's the purpose of this? does that mean each block is executed simultaneously?

The compiler would likely make only one gamemode block from the mess. Only one gameMode block is needed in a single script.

Only one script will run at a time, not simultaneously. Its a script language, not a full fledged programming language.

Thanks, so I guess it's unnecessary at best?

Yes, totally unecessary.
You would use the value of a variable to determine if you want to execute sections of the gameMode block or not.