Interesting to think about if finding a database dump of messages 30 years from now would bring back the same nostalgia. I would think that the tangible aspect would have a more profound impact.
I also think the format of letters lends itself a bit better to being re-read. A single message talking about the "last little while" rather than atomic thoughts and reactions. More context about the snapshot of life it describes. Like a page from a diary that was shared with someone.
It made her very emotional and nostaligic. Ended up much more precious than she thought when she wrote them.