K&R says:
The unary + is new with the ANSI standard. It was added for symmetry with the unary -.
What is this symmetry in regards to (i.e. it's clearly not actual, geometric symmetry) and what is the significance for? Is it important in programming?
It's just something that means "in the pattern of", or "as you would reasonably expect given the established pattern". In this particular case - is the counterpart to +, though there are other pairs like this throughout C.
It means you can do +2 as well as -2 and both work. It would be odd, or asymmetric, if +2 was somehow a syntax error. In fact, in K&R C there are a lot of odd things that were later ironed out in the standardization process. This appears to have been one of them.
You don't really need a unary + operator, you can just omit the + and the code compiles fine, but by the same logic you don't need a unary - either, you can always do 0 - 5 instead of -5, though an oversight like this would seem ridiculous.