After many days of observation I can conclude that the stabilimentum (the thick white strands of silk) is used by the Orb Weaver to prevent birds and large insects from flying into the web. It's a warning sign. There are many areas on the web that have been compromised (note the gap in the web at the upper right). The Orb Weaver is always positioned behind the stabilimentum when she's not fetching her prey. She is a very clever architect!
Did you know that a strand of spider web is 5 times stronger than the same diameter strand of steel?

Canon EOS M100
88mm, f6.3 + 10mm Macro Xtension
1/320s@ISO1000