While theyre not actively fighting, Benched Pokmon can still use their abilities and can swap out with the Active Pokmon if its in trouble.
They normal summon a banisher of Radiance or Rai-Oh, set their hand face-down, activate cards of demise, set some more cards until the field is full and end the turn with discarding 0 or maybe 1 card (if your deck doesn't like you and lets you draw into another monster)
Theyre the indisputable best in slot because they grant both haste and protection from removal
As part of a panel this morning, fans were treated to a new look at some of the cards coming to the next chapter, Fabled
It looks a bit like the Zeraora GX from Sun & Moon: L ost Thunder but the detail here and the electricity blazing off of this Mythical Pokmon is much more effective
For decks that rely on Will to win, extra redundancy like this is a big deal