The power disks/hexagons are blind-box collectibles (you never know what you're gonna get) and are sold in packs of two. They come in three flavors:
1. Abilities/Buffs. They're the red circular ones. You place them under your character and they get a bonus - right now the two that are available are Fix-It-Felix's health buff and Bolt's damage buff.
These are the only ones that matter to/can be used in playsets.
2. Aesthetic hexes. These repaint the toybox world in some way. They come in two varieties - item and skybox. Item ones reskin the ground and various other objects to fit the 'tone' of the property on the hex - for example, Nemo's item reskin paints the ground with aquarium pebbles. Skybox hexes are more ambient - they set the music for the toybox and change the skybox. I have the Alice in Wonderland skybox, and it has those sketchy trees and Caterpillar's smoky letters drifting up out of them, and plays the Unbirthday song occasionally. Again, these are Toy Box only.
3. Item hexes. Mostly vehicles and weapons - Carl's cane from up or Mulan's horse Kahn or Dumbo. Plop these hexes down in any Toy Box level and it will summon the item in question.
They're fun to collect, I guess, and the abilities ones can help make challenges easier (which in turn makes unlocking medals/Infinity Vault spins easier). But they're largely optional. I'd look at the
the website and see if you'd care about getting any of the items, abilities, or paint jobs that are out now. At $5 a pop, that's $2.50 per disk (although Toys 'R Us might still have a buy-one-get-one-for-$1 sale going on). So they're not cheap, and you don't know what you're getting. But anyone who digs collectibles or plans to spend a lot of time in the toybox would probably like them.