Shameless plug: this is what we do in EdgeDB.

I can take a guess.

PostgreSQL's JSON tooling makes it much easier to build SQL queries that return different shaped data from different tables in a single query.

The row_to_json() function for example turns an entire PostgreSQL row into a JSON object. Here's a query I built that uses that to return results from two different tables, via some CTEs and a UNION ALL: https://simonwillison.net/dashboard/row-to-json/

    with quotations as (
      select 'quotation' as type, created,
      row_to_json(blog_quotation) as row
      from blog_quotation
    ),
    blogmarks as (
      select 'blogmark' as type, created,
      row_to_json(blog_blogmark) as row
      from blog_blogmark
    ),
    combined as (
      select * from quotations
      union all
      select * from blogmarks
    )
    select * from combined order by created desc limit 100

Even more interesting is what you can do with json_agg - it lets you combine results from other tables. Here's a demo that solves the classic problem of needing to include data from a table at the end of a many-to-many relationship (in this case the tags on entries on my blog): https://simonwillison.net/dashboard/json-agg-demo/

    select
      blog_entry.id, title, slug, created,
      json_agg(json_build_object(blog_tag.id, blog_tag.tag))   
    from
      blog_entry
        join blog_entry_tags on blog_entry.id = blog_entry_tags.entry_id
        join blog_tag on blog_entry_tags.tag_id = blog_tag.id
    group by
      blog_entry.id
    order by
      blog_entry.created desc
    limit 10

(Both these demos use https://django-sql-dashboard.datasette.io/ )