U can find information on
https://www.eurotherm.com/in/glass-news-in/glass-industry-glossary-of-terms/
https://www.us.schott.com/d/tubing/ffed51fb-ea4f-47d3-972e-5a2c20f123f5/1.2/schott-brochure-technical-glasses_us.pdf
The main reasons are
To minimize the starting torque on the motor.
To load the delivery pipe network gradually if it is the only pump that feeds the network.
If it is a parallel connected pump, this method will insure against any reverse flow due to malfunctioning of upstream side check valve.
