Characters’ deaths can sometimes lose their impacts if the aftermath is filled with action and bloodshed; the letters could be a way for Carl’s legacy to continue.
The preview for the next episode does show Rick with his letter.
At least the episode shed some light on that flashforward that has been mystifying fans since the very first full Season 8 trailer debuted.
Revealing the contents later on could emotionally reconnect viewers to the tragedy of Carl’s death.