Archiving publicly available content is not illegal, otherwise sites like archive.org would have been taken down ages ago.
Users are where the content is, and most people don't have the energy to support a growing website that lacks content when another website that is full of content exists. Reddit's advantage was that people only needed one account on one website to see content related to diverse interests. Mirroring Reddit content (while being transparent about the fact that the content is mirrored) can help the Threadiverse gain this advantage and make it easier to retain users who will eventually contribute to the Threadiverse.
(In Reddit's early days, it was full of Digg crossposts too.)
The purpose of the bot is to make Reddit's content accessible without being forced to use a corporate platform. The value Reddit has, in my opinion, is the wealth of knowledge that is stored there. The content is often stale, but most of us have experienced finding a solution to a problem from a years-old Reddit thread. If you used Reddit for social interactions, this bot is not the solution for you.
Is the body of the post not appearing on certain apps or something? There is a summary that explains the bot's purpose in the post body.
You can spin up a regular instance, check "Close signups" and uncheck "Enable federation" in your admin settings, which will make your instance a private forum that is accessible from the internet.