Even when online advertising was a largely manual process, it did work, although the process was not without its fair share of pitfalls.
Notably, it was very slow and tedious. That, and the fact that your ads could be viewed by all and sundry meant the conversion rate for this method was low, which made it more expensive at the end of the day. As well, unlike programmatic media buying, brands could not take advantage of the split-second serving of advertisements to targeted users, so they could not capitalise when the market was ripe.
It is these drawbacks that programmatic online advertising has managed to do away with through AI and machine learning. Thus, companies are able to advertise faster, more accurately and for far much cheaper, resulting in higher conversions and better ROI, in the process sounding the death knell for traditional marketing methods.