Discretionary hyphens are useful when a word at the end of a line is not in your hyphenation dictionary, or when you want to break the word at a place different than that chosen by InDesign. Discretionary hyphens have the good manners to disappear when not needed. If the text is edited so that the word is no longer at the end of the line, the discretionary hyphen disappears.
A discretionary hyphen also serves another purpose: You can prevent a word from breaking by placing a discretionary hyphen in front of its first letterCmd+Shift+Hyphen (Ctrl+Shift+Hyphen). You can also prevent an individual word, or a string of words, from breaking by selecting it and then choosing No Break from the Character palette fly-out menu.
InDesign CS2 comes with 35 dictionaries representing 28 languages (there are several flavors of Englishlegal, medical, British, Canadian) and the one you have applied to your type will determine how that type is hyphenated and spellchecked. If you are working on a multilingual document, as is common in countries like Canada or Switzerland, make sure you apply the appropriate language dictionary to the appropriate passages of text. But even if you have only an excerpt, a single paragraph in a second language for example, you can specify the appropriate language dictionary for that range of text so that the text hyphenates with the right syllable breaks and so that it is spell-checked in the right language.