为什么在这种情况下需要ref

我查看了参考源，发现NumberToInt32:

http://referencesource.microsoft.com/#mscorlib/system/number.cs，a2e15b1a0c25c

[System.Security.SecuritySafeCritical]  // auto-generated
internal unsafe static Int32 ParseInt32(String s, NumberStyles style, NumberFormatInfo info) {
    Byte * numberBufferBytes = stackalloc Byte[NumberBuffer.NumberBufferBytes];
    NumberBuffer number = new NumberBuffer(numberBufferBytes);
    Int32 i = 0;
    StringToNumber(s, style, ref number, info, false);
    if ((style & NumberStyles.AllowHexSpecifier) != 0) {
        if (!HexNumberToInt32(ref number, ref i)) { 
            throw new OverflowException(Environment.GetResourceString("Overflow_Int32"));
        }
    }
    else {
        if (!NumberToInt32(ref number, ref i)) {
            throw new OverflowException(Environment.GetResourceString("Overflow_Int32"));
        }
    }
    return i;           
}
[System.Security.SecuritySafeCritical]  // auto-generated
private unsafe static Boolean NumberToInt32(ref NumberBuffer number, ref Int32 value) {
    Int32 i = number.scale;
    if (i > Int32Precision || i < number.precision) {
        return false;
    }
    char * p = number.digits;
    Contract.Assert(p != null, "");
    Int32 n = 0;
    while (--i >= 0) {
        if ((UInt32)n > (0x7FFFFFFF / 10)) {
            return false;
        }
        n *= 10;
        if (*p != ''0') {
            n += (Int32)(*p++ - '0');
        }
    }
    if (number.sign) {
        n = -n;
        if (n > 0) {
            return false;
        }
    }
    else {
        if (n < 0) {
            return false;
        }
    }
    value = n;
    return true;
}

NumberToInt32使用ref关键字进行编号的原因是什么？函数中的数字似乎没有变化。