Function insert_from_world 

pub fn insert_from_world<T>(mode: InsertMode) -> impl EntityCommand
where
    T: Component + FromWorld,

An EntityCommand that adds a component to an entity using the component’s FromWorld implementation.

T::from_world will only be invoked if the component will actually be inserted. In other words, T::from_world will not be invoked if mode is InsertMode::Keep and the entity already has the component.