Theyre part of an elaborate game with a complex set of rules, and any card thats going to give a competitive edge in a game is likely to be valuable
It forces an opponent to block it if they are able to, and so, with the right kind of Equipment to give it abilities like first strike and deathtouch, it makes every swing into a massive threat
Proliferate in ONE was restricted to single-use triggers (except for Tainted Observer, a few mythics, and the reprinted Thrummingbird), which strikes me as a good start
Each Collector Booster includes one rare or mythic with extended art, one foil mythic or rare, nine foil commons and uncommons, three special-frame cards (like showcase frames and Gilded foils), and one ancillary card (from a planeswalker deck or Commander deck, typically)