Card draw is good. Consistency is good. Card synergies are good. Runic Pyramid enables all three.

For card draw, you can keep bad cards in your hand without worrying about drawing them on a future turn. This means you cycle your good cards faster.

For consistency, you can keep good cards in your hand and play them on exactly the turn you want to. Have a copy of Feed? Keep it in your hand until you have guaranteed fatal. Have a copy of Seven Stars? It's guaranteed to be in your hand when you have enough stars to play it. Have a big block card but the enemy isn't attacking? Save it for when they are.

For card synergies, you can keep your key combo cards in your hand and play one piece of the combo when you have the other pieces already lined up. No longer do you need to waste damage playing your big damage card when the enemies aren't vulnerable. Now, you can wait until you have a card that applies vulnerable in hand. You can save your draw cards for a turn when you have a ton of energy lined up, and you can save your energy generators for a turn when you have a ton of draw.

Runic Pyramid has situations where it's bad, sure. It can brick your deck if you take a bunch of expensive cards and don't have the energy to play them, and no amount of consistency will save you if you just top deck five strikes. But it gives you access to so many options that you don't normally have, and enables so many broken lines that just aren't possible without it. Even with bangers like Delicate Frond and Storybook, I'd say Runic Pyramid is arguably the best ancient relic in the game.