If you are a web-developer, building application that store username, email, password and all that stuff, where you'd have to do multiple row level fetch of the data with a lot of parallel connections then you'd use postgres or the like.

If you are a data analyst, analysing a lot of data, which isn't updated in real time, where you'd have to do joins, aggregates which are usually column wise functions, you'd use something like a OLAP db, where duckdb is great for that!