Does the unary + operator have any practical use?

char ch = 'a';
std::cout << ch << '\n';
std::cout << +ch << '\n';

The first insertion writes the character a to cout. The second insertion writes the numeric value of ch to cout. But that's a bit obscure; it relies on the compiler applying integral promotions for the + operator.

Symmetry with unary - isn't entirely useless; it can be used for emphasis:

const int foo = -1;
const int bar = +1;

And an overloaded unary + can be used to denote an operation that yields the same logical value as its operand, while performing some non-trivial computation. (I've seen this done for type conversions in Ada, which permits unary +, but not conversions, to be overloaded.) I don't have a good C++ example to hand, and one could argue that it would be poor style. (Then again, I've seen plenty of rants about overloading <<.)

As for why C++ has it, it's probably largely for consistency with C, which added it with the 1989 ANSI standard. The C Rationale just says:

Unary plus was adopted by the C89 Committee from several implementations, for symmetry with unary minus.