Proxies are just printed playtest versions of cards that help you try out new deck ideas before you spend money on actual cards
On an empty board she makes beaters, with topdeck mode she can haste-enable your creature to hit first, she can kill or trample out of a locked board state, and more
Amethyst cards are more focused on magical themes, with perks like drawing extra cards or even returning banished cards to a player's hand through resurrection spells
The key is that its ability is much worse than either ability would be individually both destroying a land and dealing two damage a turn would be broken, but you get their choice of it, and its a choice they get to make every turn, so its not as broken as it might appear initially but still strong
COLLECTOR BOOSTER-EXCLUSIVE TREATMENTSCollector Boosters are the only place you can find certain coveted card styles, including the possibility of extended-art cards or even a card with the special Textured Foil Double Exposure treatment