The cords should be tending to close naturally and under the singers control. (isolation)
We have made it clear that the voice is produced based on the vibrations excited by the vocal cords. It is essential the cords are closed - i.e. vibrating against each other - , otherwise the vibration on which the voice is based will not be strong. The rule is GIGO applies : Garbage in, garbage out. We cannot make strong voice from weak vibration.
There is no artificial or mechanical instrument reed that operates like the vocal cords. The cords change weight and tension to create pitch.
You will remember from the chapter on sound that Timbre is perceived by the ear based on the combination of partials (overtones or harmonics) in a sound. Strong cord closure provides a source vibration that is rich in partials.
Weak cord closure may result largely from throat tension and/or incorrectly produced vowel sounds - you probably realize by now that these two issues are two sides of the same coin.
Once the throat is relaxed the cords will close naturally. However advanced singing requires a further step into control of the cord closure: we refer to cord closure as 'lateral compression'. As we increase lateral compression, the vibration produced become richer in partials, so allowing us to style a bolder sound.
There are a few simple exercises to gain and improve control of cord closure, but these are normally only done once the throat tension is substantially eliminated.