In either case there are parties who want to read those files. If material on a disk has economic, political or even criminal value then some party would like to be able to read the disk. The US government is very concerned about criminal uses of encryption but has been unsuccessful so far in making 'hard' encryption illegal. Our government has suggested that we all share our encryption keys and passwords 'in escrow' with our government for our own protection. Banks and businesses have traditionally have used password protection and encryption to maintain privacy when there is economic value in digital information like bank transfers.
Passwords are like keys, and once opened a door stays open until locked.
ATM machines automatically shut the electronic door at the end of the transaction, and if we remember to log out, our computers shut an electronic door. This is a strong endorsement by itself. The third reason has to do with the nature of the program, and I will simply refer the reader to Introduction to Cryptology by Phil Zimmerman who does a delightful job of making the opaque at least translucent. The Introduction comes free with all versions of PGP as part of the zip file. PGP is really a suite of tools to solve different problems.