A digest can be used to perform several tasks:

• in non-cryptographic applications (e.g., the index of hash tables, or a fingerprint used to detect duplicate data or to uniquely identify files)
• verify message integrity (a tampered message will have a different hash)
• store passwords so that they can't be retrieved, but can still be checked (To do this securely, you also need to salt the password.)
• generate pseudo-random numbers
• generate keys

It is critical to choose the proper hash function for your use case to avoid collisions and predictability.