Need I really use another template in this case

template<class Key, class Value>
class ThreadSafeMap
{
    std::mutex m_;
    std::map<Key, Value> c_;

public:
    Value get(Key const& k) {
        std::unique_lock<decltype(m_)> lock(m_);
        return c_[k]; // Return a copy.
    }

    template<class Value2>
    void set(Key const& k, Value2&& v) {
        std::unique_lock<decltype(m_)> lock(m_);
        c_[k] = std::forward<Value2>(v);
    }
};

Thread safe std::map: Locking the entire map and individual values

I just read the link above and found the piece of code. It looks great with it to make a thread-safe map. I understand that in the function set, we used the reference collapsing rules on the second parameter v. But I don't understand why we need import another template<class Value2>, why can't we simply use the existing template class Value?