Why do i need to use javac to compile my code?

Let's say i have a code like this :

public class Test {
    public static void main(String args[])
    {
        int x = 5;
        // Widening Casting
        double myNum = x;
        System.out.println(x + " " + myNum);
    }
}

Now i write the following commands in powershell changing the value of x from 5 to 6 once in my code and saving it, it works perfectly fine. But tutorials online suggest me to use javac for compilation. Why so?