These are great examples of epic similes which are very characteristic of Homer's work. These epic similes are detailed comparisons which are extended and are many lines in length. Homer uses a series of epic similes here to emphasise the massive number of troops in Agamemnon's army, and to highlight the very real threat to Troy. Note for example, how the first simile is constructed:
As a raging fire lights the endless forest on a high mountain peak, and the glare is seen from afar, so, as they marched, the glittering light flashed from their gleaming bronze through the sky to heaven.
The comparison of the light reflecting from the army's "gleaming bronze" to a "raging fire" that lights up a massive forest highlights the sheer number of troops that is in the Greek army and that Agamemnon has control over, and serves to raise the tension of the text at this point, as the conflict between Agamemnon's forces and Troy has not yet started. The other epic similes quoted in this question act in a similar way, focusing as they do on the numbers of troops that Agamemnon has control over. The epic similes at this point in the text therefore are used to emphasise the massive military threat that Agamemnon's forces represent to Troy, and also help to raise the tension and suspense as we think about what is going to happen next.