Manual rotation of water pump before starting the engine. Is it necessary?

Today, I came across this article while surfing through Gates website:
https://www.gatestechzone.com/en/news/2017-06-water-pump-replacement
It mentions some points about installing a new water pump. The 5th one says:
A lot of mechanics stick to the following procedure: replace the water pump, tighten the bolts, install the belt, tighten the tensioner, refill the cooling system … and start the engine (or just rev it up). But coolant takes a little time to get everywhere it’s supposed to be, so the water pump runs dry for a few seconds. This ruins the seal and heightens the risk of premature leaks or a noisy water pump. Instead we’d advise you to ease the car down after you’ve installed a new water pump, refill the cooling system, lift the car back up and manually rotate the pump about ten times, all the while making sure it rotates freely. Due to this rotating movement, coolant gets sucked into the mechanical seal component, effectively creating a film, which keeps coolant from spilling out.
I don’t understand this paragraph.
It says the pump runs dry for a few seconds. How is this possible? Isn’t the pump immersed in coolant when you refill and bleed the system? Why should we rotate it manually?