Support. Say you have 200 elite clue caskets saved up.
As you open them, you have a 1/5 chance of getting a master clue scroll.
However, after you get 1 master clue scroll, you cannot get another (from opening caskets even if you drop it on the floor).
This means you need to do the master clue (before opening another elite casket) or wait for the dropped master clue to despawn.
Adding master clue boxes would fix this. From opening 200 elite caskets, players would get ~40 master clues boxes that they can save for later (when they are no longer on clue boosters).
For those that argue this feature isn't on OSRS, that is true. Stacking clues is not on OSRS for any tier clue scroll. However, it is on Alora for easy, medium, hard, and elite clues. Therefor it makes sense to keep it consistent and also allow master clues stacking.