When should you import a post's reactions?
A post that did well, yours or someone else's, gathers people interested in a precise topic. By pasting its link, you import those who liked it, commented on it, or both. It is the one-off version of the competitors' audience: useful for a specific post, without following an account over time.
For people reacting to your own post, the first message can mention their reaction if you tick the option. It then opens on that gesture instead of pretending to have found the profile by chance.