Hi Jomano,
Welcome again.
In Shikantaza, we sit with eyes about half or 1/3 open for several reasons. First, we are not running away, nor running toward, the world, and just see what is seen without judgement or mentally becoming entangled with it. Most meditation takes us away from the outside world, but we believe that there is not "inside" or "outside" that are apart.
We also do not seek deep concentration states, deep calm, states of bliss, other unusual mental states (they happen however, and it is fine and we cherish when they happen ... but we neither run toward nor away from them either). We sit in the equanimity and wholeness of Zazen, complete as it is, without demands.
Next, it makes it easier to take this practice off the cushion into our active lives, where we don't expect to suddenly close our eyes and be removed from it all. The stillness and equanimity of Zazen is present in our bones even as, in daily life, we witness all the complexity and noise of the busy world. Stillness that is chaos, chaos just stillness, silence in all the noise.
It also is a way to help stay alert, without falling asleep.
So, Shikantaza is a little unlike what is usually called "meditation." I use the car driving example to better explain (hopefully, you don't close your eyes then either!
) ...
Drivin' Dogen - Understanding "Open Spacious Awareness"
https://www.treeleaf.org/forums/show...ious-Awareness
Gassho, Jundo
STLah