Let's say you make a niche post about a programming feature
2-6 hours later someone copies your post exactly but doesn't cross post. Now people browsing by New see the copy first and if they do see the original it feels redundant/ spammy.
If people do talk about this niche thing it's likely they won't be talking in the same place.
You as the poster won't see the engagement with your content, you won't see comments, and you may be less incentivized to post again