It provides context and meaning to posts - just think of all the disambiguation pages on Wikipedia. Are you looking for Predator the film, the UAV, etc? Really only tags can help there - multitags or disambiguation? Or both? There are discussions to be had.
It allows more depth to things like communities - I was just talking to someone who had been failing to find a place to talk about a specific British comic until they found !britishcomics. With tags, I could have tagged the community with #dandy, #beano, #2000AD, #bunty, etc.
It also provides a way for Lemmy to integrate better with the Fediverse as it doesn't currently play well with services outside the Threadiverse.
My current thinking is leaning towards FediTags - a separate, federated plug-in service. So you could run it with Lemmy, Pixelfed, etc and it would bring together all the tagged content. You look up stonehenge + photographs and get pictures on Pixelfed, Mastodon and kbin. As a plug-in the changes in code needed to, for example, Lemmy would be minimal and, hopefully, a one-time thing. Development of FediTag could then take place completely separately.