(you already have the answer)
An elevated ridge of cerebral tissue inward folds of cerebral tissue is called a GYRUS.
Gyrus are usually surrounded by sulci( depressions).
They exist because it allows the human, and other mammalian brains, to have a larger surface area. Since the brain is confined to the skull and the that space is limited
this will allow more cortical surface
area, and greater cognitive function to exist.
