Capitalizing the first letter of each beginning word in a line of poetry is traditional, if not contemporary and common. Historically, this is how poetry has been distinguished from other art forms when rendered on the page, and writing it this way is still often taught in elementary and secondary schools. In scholarship, of course, it is de rigueur that one be careful to note this capitalization, and to reproduce it faithfully when quoting.
In professional contemporary letters, however, the waters have been muddied. As a reaction to tradition, with plenty of examples even within the tradition, American poets often stopped capitalizing their lines beginning loosely with the second half of the 20th Century, a period generally associated with free verse. The abandonment of this particular custom has become the ready practice, so much so that contemporary readers now encountering capitalized first words in lines may find them startling.
Why poets even did this has essentially been lost to us, beyond the historicity of being able to say that poets just always did this. The original truth of its why may be as simple as housekeeping--poetry like this, prose like that. Or it may reside in some nobler ambition, such as attempting to reflect a studied anticipation at the great orator's next line. If the line was delivered in appropriately dramatic fashion, the capital letter in this circumstance became a cue to the reader that a deep breath was taken at this place.
More from Alberto Rios of Arizona State University.