The Trusted List is WikiTree's Watchlist. Without it, you won't be notified of any changes that you'd like to monitor (except through more generic surname tags).
But the Trusted list also allows more access to otherwise private profiles, and allows one to change the Last Name at Birth (if necessary).
But its difficult to add profiles to your Watchlist, because you have to ask permission to be added to the Trusted List, and those requests often go unnoticed or unanswered (for various reasons).
Otherwise, as already mentioned, its not necessary to be on the Trusted List in order to make changes and updates to open privacy profiles.