The idea is very simple, yet extremely useful - let holders contain multiple areas, from which one would be randomly selected. This is basically the same mechanic that is already implemented in Run Command.

It would allow for bigger randomization of the structures, such as a secret room inside a house, which is sometimes a diamond stash, sometimes an art gallery and sometimes a hidden dirt collection.

I know that it can be achieved using local triggers and conditions, but that would require extensive logic and extreme amount of variables (especially since each variable can only be 0 or 1, and not an actual integer).

For anything with more complex room setups, I'd definitely recommend opting for structure list spawning (with a structure script) rather than a holder block. They were designed for exactly this use-case! :)
Those holders are meant to be used for simple cases, but not for anything more than that.