Why __declspec(dllimport) is not needed to link against dynamic library? Where would I put it?

Question

I am new on windows and trying to understand how __declspec(dllimport) and __declspec(dllexport) work. To do this, I created a solution with two projects : MathLibrary and MathClient. The MathLibrary project contains

MathLibrary.h

#pragma once

namespace MathLibrary
{
    class __declspec(dllexport) Arithmetic
    {
    public:
        // Returns a + b
        static double Add(double a, double b);

        // Returns a - b
        static double Subtract(double a, double b);

        // Returns a * b
        static double Multiply(double a, double b);

        // Returns a / b
        static double Divide(double a, double b);
    };
}

MathLibrary.cpp

// MathLibrary.cpp : Defines the functions for the static library.
//

#include "pch.h"
//#include "framework.h"
#include "MathLibrary.h"

// TODO: This is an example of a library function

namespace MathLibrary
{
    double Arithmetic::Add(double a, double b)
    {
        return a + b;
    }

    double Arithmetic::Subtract(double a, double b)
    {
        return a - b;
    }

    double Arithmetic::Multiply(double a, double b)
    {
        return a * b;
    }

    double Arithmetic::Divide(double a, double b)
    {
        return a / b;
    }
}

The MathClient projects contains

main.cpp

#include "MathLibrary.h"
#include <iostream>

using namespace std;

int main()
{
    cout << MathLibrary::Arithmetic::Add(2, 3) << endl;
    return 0;
}

If I don't put the __declspec(dllexport) I get linking error, however, I don't need the __declspec(dllimport). Why is it not needed and where would I actually put it in the code?