"His letters are not simply a wonderful adjunct to his poems, but a vital and valuable part of them: they often serve as testing grounds for his theories and ideas, and always blend spontaneity and calculation in a way which allows us to see him in the round."
There are many reasons why poets take up other forms of writing. Not the least is a practical aspect. John Ashbery once pointed out that he, like most poets, can only write poetry for an hour or so per day--and so what to do with the rest of the hours in a day? Poets often write prose simply for the love of writing.
Baudelaire instructs us to "Always be a poet, even in prose." Writing prose can be for some poets what it is for a specialized athlete to visit the gym--a way to stay limber and fit. But there are other, deeper needs fulfilled by supplementing poetry with prose. Keats's letter writing is analogous to the modern phenomenon of poet-bloggers. And clearly, there are some timeless impulses held in common between the two.
One is the need for directness. Andrew Motion points out that "in his poems Keats cultivates a language which is carefully distanced from normal discourse. In his letters he writes with brilliant directness." The gap has closed in most modern poetry between the diction of poetry and the diction of direct address (and now some poets even experiment with Tweets or, like Paul Muldoon, craft poems in the form of text messages). Yet despite the plainspoken nature of contemporary poetry, the art of poem craft differs considerably from impromptu direct address. Poetry is inherently self-conscious in that it is word-conscious and form-conscious--even in free verse.
By contrast, a letter from John Keats would contain, according to Motion, "freely associating inquiry and incomparable verve and dash--a headlong charge in which jokes, anecdotes, 'little bits of news', snatches of bawdy, imitations of comic Shakespearian garrulity, mockery and gossip are swirled together with poetic 'axioms' and subtle deliberations." This sounds a lot like blogging to me.
Another need the epistle (be it letter or blog) fulfills is the motivation of audience. Poetry can be a lonely art. Modern audiences represent a fraction of the general populace, and even communities of fellow poets have divided into thin aesthetic slivers within this already small pie. Don Marquis is credited with saying that "publishing a volume of verse is like dropping a rose-petal down the Grand Canyon, and waiting for the echo." Writing a letter, or publishing a blog, is far more certain to draw an audible response.
And so, many poets fulfill their deeper needs for directness, engagement, and a sense of audience through epistolary communication--be it sealed in an envelope, rolled in a bottle, tied to a pigeon toe, or uploaded--like this!--to a blog.