There are many other uses of footnotes as well. The following is a brief selection, from a handout I prepared for graduate students at Indiana University in 1998:
FUN THINGS TO DO WITH FOOTNOTES*
1. Pre-emptive strike (self-defense)
Here you use the footnote to guess what objections to your line of argument the reader might bring up, and to refute or defuse them. ("Yes, I thought of that, but I don't think it holds up and here's why.")
2. Information service (bibliography)
Offer leads to the reader on what interesting and useful literature exists concerning the subject you're discussing ("Here's an interesting item you might want to read . . .").
note: this type of footnote also shows that you've read the relevant literature yourself, and that your reasoning and line of argument is therefore based on a solid awareness of what other people (and primary sources) have to say on the subject.
3. Sidelights for specialists
Here you can put information that would not be of interest to your main target audience (incidentally: a key part of writing a good research paper or book is to be sure who your audience is!), but would be of great interest to certain readers. It's better to put such material into a footnote than in the main text, because it may be so specialized that it will confuse your main readers, and may also disrupt the flow of the argument.
4. Musings and expressions of uncertainty
It's often best to put such statements as "It's difficult to determine . . ." and "I have no idea why . . . " into footnotes. It shows you've thought about these difficult issues, found them intractable, and would be interested to hear what other people come up with. (If you put such statements into the main text, it can have the subliminal effect of weakening the power of your argument.)
5. Requests for work by others
For instance, you can point out that no critical edition of such-and-such a text has been made, or that there is no useful study in any western language of the life and times of so-and-so, etc. Such comments can even inspire someone to write a Ph.D. thesis on the subject!
6. And finally, the standard one: sources
This one, quite simply, is to let the reader know where you got your information (or quotation) on a specific point. The format of the footnote should make it as easy as possible for the reader to go and find the book or article himself/herself.
Note: there are different schools of thought about whether it is or is not acceptable to put footnote numbers in mid-sentence (I think it's fine, sometimes even essential; some scholars think it's irritating). The basic rule is: make sure the footnote number is placed right next to the item to which it refers. Do not follow the practice (apparently advocated by copy editors at some presses) of putting footnote numbers only at the end of a paragraph, even if the note refers to the material in the first sentence! ____________
* On the history of the genre see Anthony Grafton, *The Footnote: A Curious History* (Cambridge: Harvard University Press, 1997). For a how-to-do-it guide see Frank A. Burkle-Young and Sandra R. Maley, *The Art of the Footnote* (Lanham, MD: University Press of America, 1996).
One last comment: as Richard Bowring has already mentioned, end-notes are intensely irritating to specialists (and, in line with what I have already written above, may not be appropriate even for more general readers). For my part, I avoid publishing with any press that is not willing to set the notes at the bottom of the page.
I am an almost-life-long Montanan; a baptized Catholic; an ardent Atheist; a practicing Buddhist; a lover of Wisdom. I find solitude to be as essential as air, though I am at times gregarious, and very often joyful, alone and with others. I laugh at my own jokes, love learning, and love those who truly love.... anything. I have certainty in little, and little time for those who are certain of much, though admire those whose certainty leads to service of humankind.
I have a BA and almost an MA in (Western) Philosophy, an MA in Buddhist Studies, and am working on a Ph.D. in Buddhist Ethics at the U of London.