I haven't worked in online advertising for a long time now, but when I did it was in the early days of retargeting, and the startup I was at was pivoting to doing search retargeting. But honestly, without any real metrics to justify it -- I suspect just investor interest / industry buzz.

I ran some quick numbers myself to see how the early experiments with it were doing, in regards to click through rates, and I didn't see any statistically significant improvements. I became very very skeptical -- one of the many things that led to me having a large fallout with the founder.

Now I'm sure in the intervening decade there's been improvements, and this is just one anecdotal datapoint, so I can't make strong claims about the efficacy of retargeting generally -- but I remain skeptical.