What does the unary plus operator do? There are several definitions that I have found (here and here) but I still have no idea what it would be used for. It seems like it doesn't do anything but there has be a reason for it, right?
It's there to be overloaded if you feel the need; for all predefined types it's essentially a no-op.
The practical uses of a no-op unary arithmetic operator are pretty limited, and tend to relate to the consequences of using a value in an arithmetic expression, rather than the operator itself. For example, it can be used to force widening from smaller integral types to int, or ensure that an expression's result is treated as an rvalue and therefore not compatible with a non-const reference parameter. I submit, however, that these uses are better suited to code golf than readability. :-)