When writing one often needs to add annotations, or short explanations, (properly called parenthesis—which aren't to be confused by brackets)—that last punctuation is called brackets though—to the text* [*See how many I managed to use in my opening sentence? For a proper explanation read on.].
Certainly in English there are a whole load of ways of doing this each with its own typographical style.The most common must be brackets, although probably not many know that the use of a pair of commas is also, technically, parenthetical. If you don't believe me mentally replace the commas on either side of 'technically' and see if it changes the sentence at all. In professionally laid out texts you will also often see the 'emphasis dash'—which is kind of a long hyphen—used to add a sub-clause into some writing. They're much loved by magazine sub-editors for some reason, but most people incorrectly use a hyphen in its place† [†Or increasingly two hyphens, which just happens to be the FOST.3™ shortcut for an emphasis dash.].
There is one other method that is becoming very common at the moment. A wonderful new technology allows even more complex parenthesis than does the footnote, but unfortunately it isn't available in all publishing technologies. If you don't believe me click on any of the links in the text.
None of these are of much help if the annotation needs to be more complex or if it has to be presented along with the main text to make sense. For example, if it is a long (often rambling) explanation about some incidental point, reference, or quotation that helps in understanding the main text or serves to place it in a wider context. For this we really need to use a proper footnote.
Authors like Terry Pratchett make great use of footnotes to add colour (that's color in America) to scenes and give us often humorous insights into his Discworld. Although their use is increasing in novels, they're still not very common in anything other than non-fiction.
Other authors like Jared Diamond make good use of them as an occasional adjunct. Books like Ralph Sawyer's translation of The Art of War seem to have a footnote at the end of every sentence (and often two). When things get this drastic then choosing the right page layouts for the footnotes is very important.
I understand why publishers do it. When they lay the page out they want a nice clean layout that looks similar on each page. It makes the book look nice when you flip through it considering to buy it or not—but don't be fooled (note that emphasis dash wasn't parenthetical). All they're doing is making the book easy to read at first glance, but much, much harder when you actually have to tackle it.
The thing is, that if the footnote is worth writing then it should be worth reading. And if it's worth reading it really has to be on the same page as the text it refers to. Reading with two bookmarks is just far too tedious.