"On First Looking into Chapman's Homer" is a poem about poetry. That may seem kind of a cop out (like, maybe Keats has run out of ideas at this point), but actually it's pretty common for John Keats. And he had his reasons, so don't be too down on him. At his young age, he was trying to figure out what he wanted to do with his life, and in committing himself to poetry, he had to find ways to justify its greatness. As we see from the poem, poetry is as important as discovering new planets or oceans.
Pump the breaks, pal. Keats is exaggerating the influence of poetry—it's good for entertainment value, but it doesn't have any real power.
Nope—Keats is right on, here. Artists have a special role in society. It's their job to show us things that we aren't able to see on our own, about our world and ourselves.