如何更好的理解const限定符？

有一段简单的C++程序：
#include<iostream>
int main()
{
    using namespace std;
    cout.setf(ios_base::fixed,ios_base::floatfield);  //fixed-point
    float tub=10.0/3.0;  //good to about 6 places
    double mint=10.0/3.0;  //good to about 15 places
    const float million=1.0e6;

    cout<<"tub= "<<tub;
    cout<<",a million tub= "<<million*tub;
    cout<<",\nand ten million tubs= ";
    cout<<10*million*tub<<endl;

    cout<<"mint= "<<mint<<" and a million mints= ";
    cout<<million*mint<<endl;
    return 0;
}


问题1：为什么使用const？
问题2：a million tubs为什么=3333333.250000??