Training Grounds can reduce the part of an activation cost represented by generic mana symbols down to nothing, as long as it still costs at least one mana
not every card from the Commander Deck will make it to Arena, but every legendary creature of the Commander Deck will head there, and various bundles come with Mythic Turtles, special sleeves, and more
Maybe you hope that this is the one curse that keeps you alive while the blue-heavy players unload on each other
Players could also get special versions of cards for playing in or doing well in tournaments, like Mutavault
What I care about the most, however, is that he is a fun commander for Thopter kindred deckswhich is such a weird niche
Warden is there to make a Serra Angel token when needed