It is clear from the beginning of "Do Not Go Gentle into That Good Night" that the speaker is addressing someone who is dying. The reference to old age in the first stanza suggests that the addressee is not dying young but has reached a stage of life when death seems natural. The fact that different types of men are referenced throughout the poem further suggests that the addressee is male. It is only in the final stanza, however, that the speaker specifies that the immediate addressee of the poem is his father.
One of the reasons for holding back this information to the end of the poem is that the feelings it expresses are universally applicable. Anyone reading the poem is likely to have experienced the death of a loved one, and those who have not will do so before long. Death is a universal subject, and the speaker's viewpoint is a novel one. The dying and those around them are often advised to accept the inevitable with serenity and dignity. Raging and raving are not encouraged. However, when someone you love is dying, even more than when you yourself are dying, you may very well feel like raging and raving, and the poem articulates this sentiment for all readers.