You may have heard that Riot is going to be bringing new and exciting changes to PBE in order to test a few things, some may be huge nerfs or buffs; others could be game play/type changes that will never see the light of day.
I myself would like to see one of these things be a "Sandbox" Game mode in custom games that allows us to set the rules for the game. e.g. - setting the amount of gold we start the game with, or the level our champions start at. I myself like to test and try and figure out bugs in a custom game, but for me, it takes so much time just to even get close to the level and items I need to try and reproduce a bug before I either run out of mana, get to the level I need (level 6), or get the amount of gold I need to test a bug with an item (Blade of the Ruined King active)
If this were to be put onto PBE only then I feel like the custom games that Riot wants to keep in rotation wouldn't be necessarily taken over given the lower amount of PBE testers and the nature of PBE itself.
I think plenty of things could be found if this game type were made. I'm sure bugs would come along with it, but we could find them easier!
TL;DR - Sandbox mode so that we don't have to grind out the gold/exp/mana regen needed to find and reproduce bugs.