I think plagiarising yourself should be fine because, in the end, it is your content and you haven't stolen it from someone. While I do like the term, I don't think repurposing your content really is plagiarism (because of the aforementioned reason).

Repurposing content is an established technique used by content creators (albeit mostly done by converting content from one medium to another. For example, from article to video).

Diversifying the media through which you send your message can help you get a greater reach. The audience on each platform may not be following you on the other platform.

Also, while the reposted content is not new in general, it is still new to that platform. Hence, you populate that particular platform with new content, thus helping it sustain itself.

It may be hard to find time to create brand new content every time for each of the different platforms.

So reposting it in different places can be a great tool to maintain consistency and keep readers on those different platforms engaged, I feel. :)