AFAIK, It’s to solve or mitigate the “Replies from other servers may be missing” issue.

Essentially imagine you are signed in on server A, responding to or looking at a comment from server B. But people on servers C, D and E have faved, boosted or replied to that same comment. Unless you or someone on your server had followed people on the other servers, you can’t see those comments or their contributions to the boost count, unless you go to view the comment on server B’s site.

Backfilling means server A fetching those other actions from other servers somehow, so that they will show up when you view it from your own server reliably. Examples of that somehow could be, obtaining all the info from server B (localized single source of truth), it could be collected individually from other servers, from a centralized server, or other means.