Each theme has a small number of possible card sets on that theme, distributed on a rarity basis, such that the specific booster that a player purchases will still be a random selection
If you can play The Emperor of Palamecia on turn 2, then you get to cast your first 4-drop spell on turn 3 and start the engine right away, which is bound to be awesome
With trainer-Pokmon combos something of a rarity in the franchise, a unique variation like Lilles Clefairy ex secret rare will very likely be a card of strong market standing
Leonin Arbiter provides a very cheap, symmetrical version of the effect
Cast it for X = 1 and throw it at someone with a bigger blocker to make sure you have that extra mana for next turn
Cards are ordered according to their TCGPlayer Market Price, the average price you might pay for a copy of the (first edition, where applicable) cards below