Reduces the hassle of having to copy and paste from other sites for Spanish words with accents, for example. It could be added as a dropdown menu in the editor, perhaps even to the already-existing Insert dropdown; when clicked, the special characters or symbols would be added to the editor. For the symbols, here's a screenshot from Wikipedia. I can agree not all of those would likely be of use here. The only ones I can personally see being useful are the tilde, the measurements, the en-dash, the em-dash, and perhaps even the arrows for signature styling or something.