Confidential information is costly to maintain. Imagine all the 3 data states (data-in-motion, data-at-rest, data-in-use) will require technology and the underlying process to manage the authorized access and usage while denying otherwise.
Most often except a few, sensitive information will diminish its value or impact overtime. An example of the “few” is the formula of a soft drink that remains as trade secret to standout the products from its competitors.
Other than technical controls like encryption or multi-factors authentication access for digital information, there are simply regulations to protect artist work copyrights, alogrithm patent etc. that are published in public domain.
Secret government documents also have expiry date to release for public interests.
The declassification together with destruction process are therefore an important stage in the information lifecycle management process. Without these, the burden to maintain secrecy will increase over time and become unmanageable.