Like why bother convicting Signal to create a backdoor for you, when you can put your nations geniuses to find some RCE bugs in Qualcomm or Apple modems or SoCs, and spy on them that way? This way nobody's the wiser.
Here are a few uses for total surveillance:
1. Public opinion can be monitored on a much more objective level than polling provides.
2. Discontent can be identified long before potential protestors are aware of others who share their views.
3. People whose views are significantly outside the usual range can be picked out and targeted in advance of any crime, even when no crime would occur.
4. Serious attempts at labor or protest organization can be resolved into definite schedules with more basis for comparison to other events than the participants themselves have access to.
5. There would be no more electoral surprises.
From this list, you may notice that states that are presently democratic have even more of a use for this capability than the others.
Onetime pad: messages are encrypted using an key that has the same length as the message
An enigmalike machine: the enigma has some problems, but that can easily be fixed. If someone uses more complicated keys than AAA they should be safe from reading the messages.