If you find a Power Cell, you’ll begin the quest called “Ancient Armory.” When you have all 5 Power Cells, you can follow this quest to a special bunker, where you’ll unlock the Shield Weaver armor that makes Aloy almost entirely invincible.
The five Power Cells can be found at:
A merchant in the central market of Meridian sells an item called Golden Fast Travel Pack. It costs 50 metal shards, 10 fatty meat, and 1 fox skin, and it unlocks unlimited fast travel.
If you get every trophy in Horizon: Zero Dawn and unlock the platinum trophy, you will also unlock a special Horizon theme for your PS4.