In Greek mythology, why was Argus Panoptes called "All-seeing"?

Argus Panoptes (All-seeing) is a many-eyed giant in Greek mythology.

Argus Panoptes, guardian of the heifer-nymph Io and son of Arestor and probably Mycene, was a primordial giant whose epithet, "Panoptes", "all-seeing", led to his being described with multiple, often one hundred, eyes. The epithet Panoptes was applied to the Titan of the Sun, Helios, and was taken up as an epithet by Zeus, Zeus Panoptes. "In a way," Walter Burkert observes, "the power and order of Argos the city are embodied in Argos the neatherd, lord of the herd and lord of the land, whose name itself is the name of the land."

The epithet Panoptes, reflecting his mythic role, set by Hera as a very effective watchman of Io, was described in a fragment of a lost poem Aigimios, attributed to Hesiod:

And set a watcher upon her, great and strong Argus, who with four eyes looks every way. And the goddess stirred in him unwearying strength: sleep never fell upon his eyes; but he kept sure watch always.