It turns your non-Knight support creatures into Knights to benefit from synergies, and it makes Knight tokens, albeit slowly, so hooray
Let's just say the bearing rules are there
Lets just go with the top overall cards worth snagging at this time because they are the most exciting to play with
That effect helps aggressive decks churn through cards at a healthy rate, and it plays into the type of strategy Emberheart Challenger slots right into, with prowess decks often employing pump spells that target their own creatures
You have generally won if you manage to get him in at all, not just because one damage and a card is so back breaking but more because there is a very good chance that if it just happened it will be happening again the next turn too
This isn't to say that foil rares from War of the Spark are all NFC - far from it