Sometimes it's a mechanical connection, sometimes it's more to do with the story, and sometimes it's to do with the development of the set
On reveal, Gandalf the Grey felt like an oddly colored card, lacking white mana to represent the character's fondness for peace, protection and benevolence, with Izzet colors representing reckless, creative innovation and experimentation
You often want to aim for a smooth mana curve that allows you to play spells consistently throughout the game
For some reason CGC designates a difference in their pop report for their energy cards
Or have you been in a heated debate before buying a card and doubting its legitimacy for a second
It serves as the primary 'Bookend' card to the Gengar ex, creating a dual-chase dynamic reminiscent of the 'Evolving Skies' Umbreon/Rayquaza split