I noticed I’ve been given platinum for season awards.
What is the platinum used for? All the merchants trade items with gold.
Microtransactions. The skins you buy in the in-game shop. Plats are the currency you buy using real world money.
Platinum is the in-game currency for tge Blizzard Shop items. Skins, Armors, Horses, etc.
You can even buy the next Battle Pass with it (100 Plat = $1.00).