For us it's speed across the whole workflow, but if I had to pick one area, content + data analysis are tied.
The data side is wild. AI can pull from Search Console, GA4, and Tag Manager, spot what's actually moving, and then go into the site and act on it. Schema, meta, internal links, new pages, the whole thing. Stuff that used to take a strategist weeks now takes minutes.
Content + page builds are the other huge one. We spun up a 5-10 page site in under 30 minutes that's getting feedback like "best site I've seen in this industry." Same with landing pages and blog content. Quality is good if you give it strong context and a brand voice.
On the human expertise question: you don't know what you don't know. AI is basically an executioner is the way I look at it. You tell it what to do, you tell it how to improve, you correct it when it's off. If you don't know how to market, you don't know what to ask it or what good output looks like. Blind leading the blind.
So I don't think AI outperforms a good marketer. It just makes a good marketer way faster. Strategy, positioning, taste, knowing what's worth doing, that's all still human. AI removes the bottleneck between deciding and shipping, but you still need someone who knows what to decide.
The biggest unlock isn't "AI does marketing for you." It's that one person can now do the work of ten, and do it faster.