The person saving your letters may be keeping a voice that conversation never slows down enough to hold.

Writing catches a version of a person that talking rarely produces. In conversation you are interrupted, hurried, adjusting to the other face, while on paper you finish your sentences and say things in the order you meant them. Whoever keeps those pages is holding a slower and more complete you. That is often why they cannot throw them out.

Write to someone properly this year, at more length than a message allows. And keep the letters people send you rather than clearing them away. Years later they are the closest anybody gets to hearing a person at their own pace.