قَارَنَّا النُّسَخَ فِي ضَوْءِ النَّهَارِ.
We compared the copies in daylight.
If lighting conditions are relevant to the examination, fī ḍawʾi al-nahār adds useful detail about how the comparison was conducted. Its role need not be to prevent a specific mistaken inference; in an appropriate context it can illustrate tatmīm, a beneficial completion.
By contrast, bi-ghayri bayyinatin blocked a predictable overreading and therefore served iḥtirās. Both add words, but the function of the addition differs. The mention of daylight is not automatically rhetorically effective in every context; its utility must be established by the inquiry.