Albert Barnes Commentary Job 38:26

"To cause it to rain on a land where no man is; On the wilderness, wherein there is no man;" — Job 38:26 (ASV)

To cause it to rain on the earth, where no man is - This is designed to heighten the conception of the power of God. It could not be claimed that this was done by humankind, for the rain was caused to fall in the desolate regions where no one lived. In the lonely desert, in the wastes remote from the dwellings of people, the rain is sent down, evidently by the providential care of God, and far beyond the reach of human agency. There is very great beauty in this whole description of God superintending the falling rain far away from the homes of people, and in those lonely wastes pouring down the waters, that the tender herb may spring up, and the flowers bloom under his hand. All this may seem to be wasted, but it is not so in the eye of God.

Not a drop of rain falls in the sandy desert or on the barren rock, however useless it may seem, that is not seen as valuable by God, and that is not designated to accomplish some important purpose there.