[ttfkam]

Off the top of my head:

The ability to do advanced regexp replacements with a function parameter in JS far outstrips anything native in Postgres.

Array manipulation, reordering, and deduplication are worlds easier in JS than native Postgres.

Manipulating byte buffers in JS are much faster and more flexible than introspecting bytea columns in native Postgres.

JSON parsing and object transformation are much easier, faster, and clearer than native Postgres (even with the recent jsonb improvements).