Instead of writing:
SELECT a.key, a.value FROM a WHERE a.key in (SELECT b.key FROM B);
SELECT a.key, a.val FROM a LEFT SEMI JOIN b ON (a.key = b.key)
So LEFT SEMI JOIN is just allows to implement efficiently IN/EXISTS queries’ semantics in your queries.
Note that right hand side of query cannot be used in WHERE clauses – it should only be used in ON join condition.