Symbolism, Imagery, Allegory
Shadows are an important image in The Left Hand of Darkness, so keep your eyes open for when they show up.
In our society, we tend to think in terms of light and dark. Light is good; dark, not so much. This is very similar to the Orgoreyns who feel the same way (see Handdara/Yomeshta in "Symbols, Imagery, and Allegory" for more on that tidbit).
But shadows play a positive role in the novel. For example, one day while Estraven and Ai are traveling the Ice, they step "out of the tent onto nothing" where "neither he nor [Ai] cast any shadow" (18.106). This moment starts the most dangerous leg of their journey.
Without shadow and only light to guide them, the two travelers cannot tell where the crevasses are. Estraven himself almost dive bombs to his death because he can't see one. So, shadow and light both have a purpose in the novel, but they're only useful when used together, like the yin yang symbol (19.28).
While we're here, let's discuss "shifgrethor." In Karhide, manners and social decorum are dictated by what is called shifgrethor. The word comes from an old Gethenian word meaning—wait for it—shadow (18.26). Shifgrethor is used by the Karhidians to manage differences without resorting to conflict, as opposed to the Orgoreyns who be keeping it real. In a manner of speaking.
Light and Darkness
Like a shadow, shifgrethor is only useful when it goes along with some light. Shifgrethor allows Estraven to navigate the Orgoreyn system, climbing his way to the top to help Ai from behind the scenes. This is quite helpful for our human compadre.
On the other hand, shifgrethor is completely alien to Ai, so he can't recognize it for what it is. This foreignness is why Ai says of Estraven, "of all the dark, obstructive, enigmatic souls I had met in this bleak city, his was the darkest" (1.78). Ai can't recognize the value of Estraven's shadow, his shifgrethor, and he doesn't understand that it's helping him in the same way shadows will later aid him traveling the Ice.
The realization of the positive side of shadow and shifgrethor comes when Ai presents Estraven with a yin yang symbol, promoting the novel's theme of unity of opposites.