The Private Key may be used to:
Protection and safeguarding of the Private Key must be performed as the Private Key is essentially a Bearer Token.
The two keys are used in Asymmetric Key Cryptography stems from the use of different keys to perform these opposite functions, each the inverse of the other.
Asymmetric Key Cryptography as contrasted Symmetric Key Cryptography which relies on the same Private Key or Secret-key to perform both operations.