It's like taking your hands off the wheel when driving. A few seconds and you're probably okay. An hour? You're in a ditch!
Sure, but modding is something you don't need...until you absolutely do. Just takes one person who wants to spam porn/gore/etc to a subreddit/forum/etc and without a human able to ban/limit those actions, it's trivial to takeover a smaller community.
To be specific, I'm referring to in non-crazy western countries with something resembling freedom of speech, not PRC "Winnie the Pooh is banned" places.
I moderate a few small, and mostly inactive, subs. They don't see a whole lot of activity.
On the odd moments I do drop into mod mode (a few times a year, if that), what I see is a fair bit of flagged spam ... and a lot of off-topic posts. One sub in particular seems to have had its topic (a software application of use to Google+ users saving data, which was last relevant on 1 April 2019) with an exam of some sort in India.
Topic drift, relevance drift, and lightweight content displacing substantive posts are far more likely without dedicated, mission-aware mods. AskScience and AskHistorians would be most especially subject to that, as well known examples. I can think of numerous others.
The other issue would be various forms of abuse and brigading, below the thresholds which Reddit's automated systems would detect. Unmoderated forums would far more likely become unpleasant places to participate.
r/fragrance for example decided to stay open, but the mods just would quit doing anything. That'll show us!
What happened: no spam. A few silly posts. A lot of good posts. A lot of people remarking how much better the sub is without mods. Mods returned, promising to 'consider community feedback' once they realized they lost control of the narrative.