What role does the Solidity constructor play?

I can't tell the difference between test_1 and test_2. (If you create a function that calls testMessage, they both work the same way.)

 contract test_1 {
  US>//#string testMessage="Hello, world!";
}

contract test_2 {
  US>string testMessage;
    constructor(){
      testMessage="Hello, world!";
    }
}

As I mentioned earlier, when I call testMessage, both of them show Hello, world properly.

function getMessage()public view returns(string){
  return testMessage;
}

execution results:

Hello, world!

I know the constructor has a special meaning, but I don't know exactly what role it has.