Very good question. The main reason for downvotes is, that you get to counter people who post comments and just vote nothing but their own comments up. That way all their curation and voting rewards are spent on worthless comments made by themselves and the ProofOfBrain mechanics are completely fallen apart. But it also meant that was the most effective way to "mine" new Coins for yourself. There were several other abuse ways based on this very same concept, none in any way healthy for the platform.
The Downvotes were implemented to create a counter to exactly that. Everything else is just a natural evolution occurring beyond the crude implementation of the Vote/Downvote system.
The original Chain was launched without the current Downvotes, they were added in the late stages of the STEEM Blockchain, not long before the Hostile Takeover and the Fork into Hive. To be fair, there have been Downvotes from the start, but Upvoting and Downvoting had been the exact same Mana Pool, which made them useless from a practical perspective and we could observe them basically never used for anti-abuse. They got useful and common after getting their own Voting Mana pool. There's a side story with a step in between and free downvotes, but that's long gone and I'll just skip that.
Was this helpful for you? You can ask me to dig deeper if you want.
Very helpful. Thank you very much. Sometimes things evolve in a complicated way and after awhile people start to question the purpose and value. I like win-win situations. Thanks again.
Sure, you are very welcome, glad I could help.