Coalesce is needed because you don't know in advance if the value will be on the source side, the destination side, or both. If you only refer to one of the join keys, you will get nulls for the records that don't exist for that side. Coalesce solves this problem by using the first non-null value it finds. Does this make sense?

As an aside, you don't need coalesce for the a left join...only for a full join.