Why would I use Nimrod and not C++? C++ matured quite a bit over the last 20 years and I'm not sure what advantages Nimrod has over it. The tutorial mentions C++ a lot of times but does not mention any advantage for Nimrod.
That being said, I believe a big draw is its metaprogramming abilities. In Nimrod, metaprogramming is done using Nimrod. In C++ you have the very limited, backwards compatible to C, C++ preprocessor and template metaprogramming. C++ is progressing slowly towards metaprogramming in C++ with constexpr. constexpr support is much stronger in C++14.
D is similar. In D, the metaprogramming language is (I believe) D itself, or at least very similar to it. D also has strong support for compile time function execution.
Nimrod also does a lot more to guarantee safety. C++ can only do so much, due to its (mostly) backwards compatibility to C.
Like, imagine you could write Python instead of C++, except that it only ran 10% slower instead of 10,000% slower.