I found this article quite useful20 in describing how tags and segments work in ConvertKit, and when to use which.
There is a summary table at the end that describes it pretty well and in which situations to use tags vs. segments.
I'll be honest... I rarely use segments. But I use tags all the time.
I use the tags as flags within my automation workflows and use them in filters for custom broadcasts. This last part is the critical thing for me... segments are quite limited in automation... so I stick with tags so I can future-proof any greenfield ideas I may have in the future.