If there's no reason to suspect the password is known or can reasonably be guessed by somebody else, then there's no reason to change it.
Password is or might be known to somebody else:
- it has been stolen, or guessed, or bruteforced
- you notice strange changes in your accounts or suspicious lines in your logs, etc. so somebody might be using your password
- your machine has been infected, so any password you have typed lately might have been stolen
- it has been handed out to other people on purpose (for example for getting support, etc.)
Password can reasonably be guessed (weak or weakened password):
- an attacker has stolen a hash of your password, or they have stolen a file encrypted with your password (they can bruteforce it more easily with dedicated hardware and software)
- you use a weak password, or even use the default password provided by the service or device
- you are forced to use a weak password (like a numeric PIN) which could be bruteforced within a reasonably short time, so you need to change it regularly to make it hard for the attacker to gain continuous access
- you use the same password for two or more different purposes, and that makes it more likely to be stolen or guessed
So for an encrypted HDD, for example, this means you should change the password if you suspect your nosy coworker who is always glancing at your keyboard has seen it or has finally been able to figure it out. Or for example, you should change it if somebody somehow stole a copy of your HDD (it could also be a backup copy), because then the attacker can try to bruteforce it much more easily (with specific software or hardware). If they finally find the password they will be able to read all your data in the stolen HDD copy, but at least they can't come back and use that same password to easily read all your HDD again (because you have changed the password). Yet another example: you should change your HDD password if you are using the same password as your WIFI, and you are giving your WIFI password to all your friends so they can access the internet at your home. Of course those passwords should be different.