It takes a while before a deck with the number of nonbasic lands tri-color decks would naturally play would get to stop sacrificing lands, and many of even your similarly colored opponents will be able to keep operating for a similar amount of time unless you bust this out right away
On top of giving near immunity to counters, red and black have a ton of creature and artifact removal effects, and with Shadow giving them split second, you can get around most tricks that other colors can use to protect their permanents
Like most Magic: The Gathering cards, the value of Arena Athlete depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Foundations Jumpstart)
As for the Fabled set's champ card, fans will be excited to see Maleficent return in all her glory
Not to mention, it allows you to add more cards into your hand if you make use of its Ohana ability