Art can preserve a feeling after circumstances disappear.

A song written in one particular summer keeps that summer available to strangers decades later, long after everyone involved has moved on. The circumstances are gone and the feeling has been given a container that does not decay with them.

This is worth using at a personal scale. Write the letter, take the photograph, record the voice. Not for posterity in any grand sense, only so a feeling you currently have will still be reachable after the situation that produced it has dissolved.