I would have to agree that not all poetry is a celebration, sometimes it is somewhat more serious than that or sometimes it is sad and is a lament. I think that Hesiod is talking about how powerful poetry can be and that he sees it that way. He sees it as joyous and as something that even the gods would enjoy in their daily lives. So we humans ought to enjoy it too. It is both.
Later poets do the same thing, for example, Homer.
When he does this, he is claiming for himself the power of this inspiration. To call it divine or to call it something from the muses is partly a serious claim about what he believed he experienced, a vision of some sort, but it is also a metaphorical claim about his own powers as a creative writer. I myself would not say that Hesiod every really saw some goddesses but he probably did have an experience of some sort that was lofty and uplifting and that gave him his ideas. He called that the experience of encountering these goddesses. Later poets had similar experiences.