To mark linewrapping-breakpoints in strings.

To prevent ligatures from forming when you need that.

if you write كلب which is an arabic word written right to left in the middle of an english sentence, you want to preserve the order of the characters in the stream for computer processing purposes. meaning the chararacter ك must come before the ل and after the e and the space with respect to the memory layout. whereas when displayed, it must be inverted to be legible. the solution is to have an invisible character that indicates a switch in text direction. if you were wondering, the situation where you want to write text in a foreign language within your text is very common outside english speaking countries.