What is the purpose of a declaration like int (x); or int (x) = 10;

by

The fact that this rule is applicable in your case is not deliberate: It’s ultimately a result of keeping the grammar simple. There is no incentive to prohibit declarations such as yours, but there are great disincentives to complicate rules, especially if those are intricate as they are.

In short, if you don’t want to use this needlessly obfuscated syntax, don’t.
C++ rarely forces you to write readable code.

Surprisingly there are scenarios in which parentheses can save the day, though:

std::string foo();

namespace detail
{
    int foo(long); // Another foo

    struct Bar
    {
        friend std::string ::foo(); // Doesn't compile for obvious reasons.

        friend std::string (::foo)(); // Voilà!
    };
}

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