The optional hyphen is usually invisible. The optional hyphen marks where a word should be hyphenated if the word lands at the end of a line. You can put multiple optional hyphens in a word.
Why would you want to do this? Some words bedevil hyphenation engines. For instance, TrueType will often get hyphenated as Tru-eType. To prevent this, I put an optional hyphen in the middle (True~Type) so it will be hyphenated correctly.
How do you know whether a word won’t be hyphenated correctly? The problem usually afflicts words that aren’t in a standard hyphenation dictionary, like jargon words, unusual proper names, and other words with nonstandard spellings, like trade names. As Justice Potter Stewart might have said, you’ll know it when you see it.
I use optional hyphens pretty much as a matter of course when I’m adding hyphenation by hand for purposes of adjusting line length (in left-aligned text) or tightening up word spacing (in fully justified text). That way if the line breaks subsequently change due to revisions to the text, you aren’t left with an inappropriately hy-phen-a-ted word in the middle of a line.