This outfit does have the usual assortment of “because fantasy” elements to it but it also highlights how Cas’ general motif and design is adaptable to her armour - a leather tabard over the top of the metal plate means she can boldly declare she’s a Seeker without having to compromise her safety.
It still has layers (quilting, chain, plate, decoration) and it does have a few oddities (like defending her muscles rather than her joints) they’re not really related to it’s flamboyance. Overall it conveys that she’s there to kick ass and Seek... something.
It’s also really good to see the movement away from over-exaggerated plate design (which mostly has it’s basis in stylizing for wargame minatures) and towards somewhat more realistic amounts of metal for a person to wear and still engage in mortal combat.
As a side note it is true that tabard’s did mostly fall by the wayside once plate became the core of the armour (rather than an augmentation to chainmail armour), this was largely due to fashion and the desire to show off the quality of the plate. Priorities in your own world may vary.
- wincenworks