Board Thread:Off Topic/@comment-27144409-20190627122349/@comment-29684190-20190722141628

I believe that's because instruments, especially strings naturally become out of tune over time. And if you have a guitar in a case it presses against the strings which slowly go out of tune.

Strings are under such pressure that there isn't a material that could keep that up without degrading over time.

Tuning your instrument is just to check up if everything's okay. The last thing you want to do is start playing and find out you're off.