Peter Ackroyd writes in his biography of T. S. Eliot that there were two reasons for including footnotes (pages 177-178 of the German translation):
- The first motivation was to avoid accusations of plagiarism, since the poem quotes from and alludes to various other works, both in English and in other languages.
- The second motivation was that the poem didn't seem long enough to be published in book form. The addition of the footnotes led to what Eliot later called "the remarkable exposition of bogus scholarship that we still see today" (in On Poetry and Poets, 1948).
The versions printed in the literary magazines The Dial (USA) and The Criterion (UK, edited by T. S. Eliot) didn't have the footnotes. Modern editions of the poem typically reproduce the footnotes.
The majority of the notes don't really steer interpretation in a specific direction. They identify sources and backgrounds. Some of them aren't even of any use, for example, when Eliot writes about the line "With a dead sound on the final stroke of nine" (line 68), "A phenomenon which I have often observed." Or his note on line 199: "I do not know the origin of the ballad from which these lines are taken: it was reported to me from Sidney, Australia."
One effect seems to have been that the notes "left some early critics mystified enough that they couldn't come out and say they didn't like the poem for fear their ignorance of [Eliot's] learned and sophisticated methods would be discovered" (Karr, xv; emphasis by Karr).
None of the above suggests that the notes were intended to help readers. Mary Karr encourages readers to start by reading the poem "intuitively" (page xxi) and to listen to the poem's music (page xxiii). After that, you can still read the texts that Eliot quotes or alludes to. However, she adds: "Just be forewarned that such investigations may not pay off with a different or greater understanding that your own reading will. You will get no encoded punchline unavailable from the poem's surface" (page xxi).
Ackroyd, Peter: T. S. Eliot. Eine Biographie. Translated by Wolfgang Held. Suhrkamp, 1988. (English version: 1984.)
Karr, Mary: "How to Read 'The Waste Land' So It Alters Your Soul Rather Than Just Addling Your Head", in: T. S. Eliot: The Waste Land and Other Writings. New York: The Modern Library, 2001.
See also the related question, Were T. S. Eliot's notes to The Waste Land partly inspired by plagiarism laws?