Downvoting is useful for dealing with spam and attempts to abuse the system, but can also be used abusively if it isn't countered.
See here for example: https://steemit.com/earn/@tuvokhl/shit-post-5#@troglodactyl/re-tuvokhl-shit-post-5-20170709t141555576z
Without downvoting, people could loot the community by getting payouts for self voting spam.