To unlock new passages in the stage.
A small stone statue to place an item.
The Ancient Shrines (ナックルズ族の台座 Nakkuruzu-zoku no daiza?, lit. Knuckles Tribe pedestals) are a number of small shrine-like temples located in every stage in Sonic Adventure 2. They can unlock alternate paths in the different stages with the Mystic Melody.
At first, the characters can only examine the shrines (only Knuckles seems familiar with them). However, upon obtaining the Mystic Melody upgrade, they can use the music created from it to activate these shrines, and they will create a path that will lead to either short cuts through the level, other upgrades, or lost Chao.
How these paths open depends on the level such as: