Brad Allen wrote
Why would a plane do such a maneuver?

Most likely the following from the AIM

5-4-19. Side-step Maneuver

a. ATC may authorize a standard instrument approach procedure which serves
either one of parallel runways that are separated by 1,200 feet or less
followed by a straight-in landing on the adjacent runway.

b. Aircraft that will execute a side-step maneuver will be cleared for a
specified approach procedure and landing on the adjacent parallel runway.
Example, "cleared ILS runway 7 left approach, side-step to runway 7
right." Pilots are expected to commence the side-step maneuver as soon as
possible after the runway or runway environment is in sight.

NOTE-
Side-step minima are flown to a Minimum Descent Altitude (MDA) regardless
of the approach authorized.

c. Landing minimums to the adjacent runway will be based on nonprecision
criteria and therefore higher than the precision minimums to the primary
runway, but will normally be lower than the published circling minimums.

Bob Moore