In the first case, if for a given row bar.category_id <> 5 then there's no match and hence null will be returned for any value from bar. Thus there will be no match in baz either (assuming the id columns are non-null).

In the second case, the first join will always return a row from bar for a given foo_id if it exists, but the second join does not return values from baz for rows where bar.category_id <> 5.