The only possible benefit to this kind of behavior is creating the impression that there's more traffic on Reddit than there really is, from which only Reddit benefits.
There's plenty of other reasons to do this. From scammers trying to legitimize accounts to use later to groups trying to sway user opinions on the site. This sort of thing has been going on on plenty of other websites for years. This is the same strategy the porn bots on Tumblr used, and they were so prolific there that they got the Tumblr app removed from Apple's app store.
It wouldnt be that hard to script this if you already had a bunch of accounts and were just copying existing threads. What's interesting is that there were no other people accidentally joining in at any point. So either this was done by reddit and the timestamps are all 'fake', or this was done at a weird time in a niche community with low engagement.
Either way it's either astroturfing or someone farming up the karma in their bot farm to make them more attractive for sale (to get around comment/karma minimums)