[cestith]

This is due to the multiuser nature of Unix-like systems.

/etc/localtime is set by the administrator. It may change without notice to the user.

TZ is part of the user's environment and the user sets it. All applications run by the user should honor the user's wishes if the user's not falling back to system defaults.

If you're setting TZ for yourself, your libc can update things when you update the variable and restart any applications you're running under the old value. It can therefore save cycles. If you're falling back to the system default that's not under the same user's control, then it must be ready to deal with unexpected changes.