The point is that you might have multiple aspects of valid time:

    SELECT person AS OF t1 FROM upstream AS OF t2

Which reconstructs the view that our upstream source would have constructed for a particular time (t1) from its perspective at a different time (t2).

    JOIN event_feed_1 AS OF t1 AND event_feed_2 AS OF t2

There’s various reasons you need to offset the comparison of two event feeds, such as their clocks not matching.