It's a good idea to cycle the ABS whilst flushing new fluid through. People rarely activate the ABS and brake fluid being hygroscopic, the water can corrode the ABS pump pistons, so it's well worth agitating the pistons to shift the dirt and corrosion.
Not a problem for people who stick to the 2 year brake fluid service, but people leave brake fluid in there for 10 years will have problems.
It was actually a requirement on the MK3 platform Golf. Dealers were instructed to do it from VW Germany. But these days they just don't bother unless asked.
