Epic similies as a function of epic poetry have always been implemented in order to highlight the heroic quality of the work and the characters contained therein. By linking a poem's hero to a character or creature from a past work, the author could exalt his character while drawing a comparision that would make his hero appear to be legitimate. For example, when Satan is compared to the great Leviathan in Book I, it brings to mind the supernatural eminence of the sea-serpent, as well as his destructive nature.
However, Milton uses this convention of epic poetry to point out something to the reader. Milton doesn't use a single extended simile in his description of Heaven or God, highlighting their unique and unparalleled nature. There is absolutely nothing that can compare the to ethereal splendor that is God. In contrast, many of the epic similies used in Paradise Lost refer to Satan and his angels. With some comparing ancient events and people to the Hell, Eden, and the War in Heaven. This use of the figure of speech is indicative of the physical quality of evil. That is to say, evil is easily defined in earthly terms (this echoes Dante's idea that sin and Hell are corporeal and that virtue and Heaven are intangible).