The engagement is what's valuable. You can't have engagement without content, that's true. However, content without engagement is worthless.

With that in mind, if you "steal" a post from reddit and it generates engagement over here, nobody will have any problems with that. However, if you "steal" a bunch of posts from reddit and spam them over here, they probably won't get engagement and therefore only serve to clutter the feed with empty content.

It's important to remember that Lemmy and the Fediverse is a community, just like reddit is a community. Each of those communities behaves differently and has different expectations. Once you learn the community and the expectations, it becomes a lot easier to understand what you should and should not post.