The goal of the Devotees is to help your three-color deck when you're missing one of your colors without making it too easy to splash a fourth color
"Chaos ensues on that plane" means that plane card's chaos ability will trigger
However, effects based on those costs that were paid for the original ability are copied as though those same costs were paid for the copy
Those free spells often include ramp, removal, or more creatures, so its a huge tempo swing
When they pay 2 mana and one card to kill 2 things you are very far behind
In every way, its intended to feel like a visualization of the world of Magic, without fully replacing cards with animated creatures, with the logic that that makes it feel less true to the visual identity of the game