Double strike is self-explanatory: you deal twice the damage (a creature's power)
This helps ensure your custom designs have durability and can shuffle smoothly with the rest of your deck
Check out the list earlier in this post for a look at which Charizard cards are worth big bucks

Elemental Mascot Elemental Masterpiece Elemental Summoning Emberwilde Caliph Enigma Drake Enthusiastic Mechanaut Epic Experiment Eris, Roar of the Storm Eruth, Tormented Prophet Essence Backlash Etherium-Horn Sorcerer Exalted Flamer of Tzeentch Expansion // Explosion Expansion // Explosion Experimental Overload Expressive Iteration Farideh, Devil's Chosen Fearless Swashbuckler Fell Beast's Shriek Fevered Visions Firemind's Foresight Firemind's Research Firestorm Hellkite Firkraag, Cunning Instigator Flame of Anor Flaring Cinder Fluxcharger Frenetic Efreet Frenetic Sliver Frost Fair Lure Fish Frostburn Weird Frostcliff Siege Gadget Technician Galazeth Prismari Galvanic Iteration Gandalf the Grey Gandalf's Sanction Gelectrode Ghyrson Starn, Kelermorph Gitaxian Spellstalker Glacial Dragonhunt Gleaming Geardrake Goblin Electromancer Goblin Flectomancer Goblin Test Pilot Hawkeye, Avenging Archer Heaven Sent Herald of Kozilek How to Keep an Izzet Mage Busy (playtest) Hustle // Bustle Hypersonic Dragon Hypothesizzle Ian Malcolm, Chaotician Iceman and Firestar Idris, Soul of the TARDIS Ill-Timed Explosion Improbable Alliance Inside Out Inspired Skypainter Intruding Soulrager Invade the City Invasion of Kaladesh Invasion of the Giants Invert // Invent Invert // Invent Invert Polarity Invoke the Firemind Ionize Iron Lad, Young Avenger Iron Man, Master of Machines Iron Man, Titan of Innovation Izzet Charm

The detail on Umbreon cards is something collectors love, and this artwork is no different
If Pikachu were a cult leader, this would be its calling card, and it would probably possess some strange hypnotic powers