Bridgy Backfeed sends current and future silo responses to your posts as webmentions, for your posts to handle as they come in. It would be great to have the complementary service of Bridgy Backfill (feel free to bikeshed the name) to send past silo responses to your posts as webmentions, as a way of backfilling / importing old responses to your posts, including your old posts.
The key use-case here is: help with migration away from social media.
If you can backfill/import the comments/responses on your old social media posts into your own site, then you have fewer reasons to keep around those old social media posts, and thus fewer reasons to keep around that social media account.
Such migration of responses to old social media posts would also enable deletion of those old social media posts, as a step towards de-activating and deleting a specific social media account.
Since it’s likely you may only want to run this "once" and it may take a while as it will likely require crawling your silo posts history which may be throttled, this makes sense as a separate service that you authenticate, activate, and then allow it to progress back through your old posts (and old responses to your posts), perhaps with some progress indicator, and perhaps with some sort of notification when it has completed.
This new service would be in addition to the existing Bridgy services: Bridgy Backfeed, Bridgy Publish, Bridgy for Blogs, and Bridgy Fed.