This is really important and touching for all types of situations. I have been fortunate to be the recipient of messages like that, as well as the deliverer. The warmth I felt in both occasions was overwhelming. It seems so small, but those moments can really help people in massive ways --ways that can be hard to verbalize.
Example: I had a friend tell me years afterwards about their emotional struggles and how my note helped them through a difficult and dark time in their life. Similarly, I had some very difficult days where it was hard to cope with my grief (not a loss of life in my situation). A friend had sent me a card telling me how much I meant to so many people. On my hard days I would read the card over and over. Something small, helped me in a massive way.