A lot of logic just becomes easier with javascript. If you have complex-ish trigger logic, or a procedure etc - doing these in pl/pgsql is just hard if not intracatable. Also allows up reuse of a lot of code that people have built with npm - even if not directly with require(), it becomes possible to bundle compiled binary.

I used this for storing patches of data (to save space, compression is a pain in Postgres) and applying runtime to show full version etc. It's fun.