Shadow banning is a very useful tool for moderation. It is extremely frustrating as a moderator dealing with users that persistently shit up the place and crank out new accounts to continue the abuse as soon as you ban them. Shadowbans greatly relieve the moderation workload in cases like this because the abusive users keep themselves occupied, potentially for weeks. As someone who has previously moderated a large phpbb, I don't lose any sleep over wasting the time of abusive users since they are happy to waste my time as a moderator by deliberately and repeatedly breaking the rules.