Sorin, the Mirthless is a classic card in that it fits a very traditional style
Green, keeping in line with its tendency towards creatures with high stat lines or large costs, typically grants card draw as a reward for playing high-power or high-cost creatures, keeping them on the board, dealing damage to a player, or bases its card draw off of the power of your creatures (for example, "when this creature deals damage to a player, draw cards equal to its power")
In fact, you probably solve this entire mystery by going to over a WOTC forum and asking them why they're "using the old Pokmon holofoil" they'll stumble over themselves to "Um, actually" you and you'll get your answer
Overall, the XYZ monster mechanic is one of the game's easiest to understand, and these cards have found their way into meta strategies for well over a decade at this point
Enchanted Rare cards also have a textured foil treatment, making them feel much more special than a standard foil Lorcana card