When and why do I need to use cin ignore in C

Question

In C++, I developed a simple application that requested the user to enter a number and then a string. 

Surprisingly, when I ran the application, it never paused to ask for the string. 

It simply ignored it. 

After conducting some research on StackOverflow, I discovered that I needed to include the following line:

cin.ignore(256, '\n');

before the line with the string input 

That addressed the problem and allowed the software to run. 

My issue is why C++ need the cin.ignore() line, and how can I forecast when I will need to use it.

Here's the software I created:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    double num;
    string mystr;

    cout << "Please enter a number: " << "\n";
    cin >> num;
    cout << "Your number is: " << num << "\n";
    cin.ignore(256, '\n'); // Why do I need this line?
    cout << "Please enter your name: \n";
    getline (cin, mystr);
    cout << "So your name is " << mystr << "?\n";
    cout << "Have a nice day. \n";

}