The (trivial) big picture is to allow client-side expressions in psql (which as a \if:-) by reusing pgbench expression engine, so that one could write things like
\let i :j + 12 * :k
or
\if :VERSION_NUM < 140000
I still haven't really grasped why this isn't done by embedding a client-side scripting language interpreter, giving us vastly greater capabilities with only the maintenance of the glue instead of writing our own ad-hoc scripting tool. Something confine-able like JavaScript or Lua.
