Kaldheim is strongly focused on supporting tribal decks and effects. Resplendent Marshal is an example of a pushed constructed card for tribal synergies, and that's probably the reason why it's a mythic rare.
Its main application will be seen in Angel and Warrior tribal decks, but other tribal lists using white Mana will also want a copy or two of Resplendent Marshal in the main deck.
The card is gaining traction due to it being a mythic rare, but the steam should go out as soon as the meta settles. This means that Resplendent Marshal will most likely not be able to hold the current price for too long.