Assigning watchers in Redmine?

Question

Redmine

In Redmine, it is possible to assign Observers (aka Watchers) to a task.
What is the meaning of this mechanism?
In our understanding, observers are those users who are not authors and executors, but simply have to monitor the execution of the task (for example, the head of the developer / task executor)
By default, users see only their own or the task assigned to him (Task visibility: created or assigned to the user). With this visibility value, the Observer does NOT see tasks in which it is specified as "Observer".
What am I doing wrong ?

Answer 1

observers and task visibility rights are not connected, observers are created in order to enable email notification for associated users, if the user does not see the task, then adding him to the observers will not change anything and will not affect anything.

Answer 2

There are just a few chips that are tailored for the observer. About one of them you were told above:
You can also pull out a preconfigured block on my page and set filtering conditions based on observers.
In general, the mechanism is rather strange.
You can programmatically expand the task visibility conditions.

Answer 3

How can you set the Visibility zone: your own task, the task assigned to you and the task in which you are listed as an observer?