stmt_init() seems to clear previous (possibly erroneous) results on the DB connection, which means you don't necessarily need to use it but it could make the code more robust.
In a PHPUnit test, I had a sequence of prepared queries on the same connection. One of them fetched a row from a SELECT but didn't keep fetching until it drained the connection, so it left some stale results. When the next query did this:
<?php
$db = $this->getConnection()->getDbConnection();
$preparedQuery = $db->prepare ($query);
?>
the prepare() call generated an error: "Could not prepare query: Commands out of sync; you can't run this command now." Changing to this:
<?php
$db = $this->getConnection()->getDbConnection();
$preparedQuery = $db->stmt_init();
$preparedQuery->prepare ($query);
?>
resolved the problem.