How would you justify the digressions in Lycidas?

In all his poetry, Milton leans toward the "meta." Throughout his work, we see him thinking about his role as a poet, as an inspirational voice and as one who seeks to take his place among the most esteemed poets. Quite often, his work offers a meditation on the anxiety that he may not reach his goal, that he may have started too late on his poetic quest, or that he will be cut off—by blindness or death itself—before he has immortalized himself.
In Lycidas, part of the work of the poem is a contemplation of Milton's worthiness to assume the voice of the pastoral elegy and merge his own talent with those who have already sung in this vein, or even of King, whose death prompts the poem.
While following the pattern of the classical pastoral elegy, Milton's poem does use these two digressions to address his ongoing themes. First, the initial digression contemplates the value of poetic fame and the ephemeral quality of it. Like King, Milton's voice might be cut off too soon and the labor he invested in his art wasted.
The second digression addresses the unworthy voices among the clergy who use their position improperly. By being lazy, manipulative or ambitious, they sully their voice, reducing themselves to "blind mouths." Milton presents the deceased Edward King—who was studying theology at Cambridge—and by extension himself, by contrast, as a visionary who would use the gift of poetry to lead others to Christ.