Gandalf the Grey is the quintessential fantasy wizard, so it made sense for his card design to be blue-red as those are the colors that do the most wizardy things in-game
Unlike Wellwisher this doesn't need any infrastructure to work, and it doesn't tap, so you can attack, block, grab equipment, and more
These each have unique cards and playstyles for players to experiment with
Were generally talking about the cheapest versions of the cards found in Play boosters and Collector boosters, although some of the cards can only be found on the Retro bonus sheet (Ill mention this when relevant)
When Aragorn, King of Gondor enters the battlefield, you become the monarch
Pick a theme, then for every card you're considering, ask yourself, "does this card support my theme?" If not, then you can safely cut it