These are sub-goals (though I am enumerating some of them) which is important to understand. No sub-goal is a "must" because there might be another way to implement the main goal. The main goal is that voters are confident that the system has met the other goals, particularly accuracy and precision. There are a number of possible ways to attain that trust.

Perversely, the trust goal may actually be faked so long as the other goals are attained. The trust goal is one of feeling. While some people must truly understand the correct reason why the system can be trusted, it is quite possible that many voters might trust the system for incorrect reasons. As long as everybody is not incorrect, and this universal error does not permit accuracy to fail, the system is still returning the right result.

The confidence goal means that people will treat the government as legitimate, and thus will defend it and support it and in the extreme die for it. This seems Machiavelian of course, and even comes with a risk that if people discover their trust was incorrect, they could act differently. (Though it is not so bad if you learn the system is still valid, though you didn't quite correctly understand why.)

Of course, a system everybody can understand sufficiently to have confidence in it remains the top expression of this goal. It is one of the real burdens to DRE voting machines.