What is the purpose of the Step Definition file in Cucumber?

Feature file step:
  "When the user enters username "admin" and password "Admin@123""

Step Definition method:
  @When("the user enters username {string} and password {string}")
  public void enterCredentials(String username, String password) {
      loginPage.enterUsername(username);
      loginPage.enterPassword(password);
  }

package stepDefinitions;

import io.cucumber.java.en.*;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import pages.LoginPage;
import static org.testng.Assert.*;

public class LoginSteps {

    WebDriver driver;
    LoginPage loginPage;

    @Given("the user is on the login page")
    public void navigateToLoginPage() {
        driver = new ChromeDriver();
        driver.get("https://example.com/login");
        loginPage = new LoginPage(driver);
    }

    @When("the user enters username {string} and password {string}")
    public void enterCredentials(String username, String password) {
        loginPage.enterUsername(username);
        loginPage.enterPassword(password);
    }

    @When("the user clicks the Login button")
    public void clickLogin() {
        loginPage.clickLoginButton();
    }

    @Then("the user should be redirected to the dashboard")
    public void verifyDashboard() {
        assertTrue(driver.getTitle().contains("Dashboard"));
    }

    @After
    public void tearDown() {
        if (driver != null) driver.quit();
    }
}

// Cucumber Expression (recommended, simpler)
@When("the user enters {string} into the {string} field")
public void enterText(String value, String field) { }

// Regex (more powerful)
@When("^the user enters \"([^\"]*)\" into the \"([^\"]*)\" field$")
public void enterText(String value, String field) { }

src/
  test/
    java/
      stepDefinitions/
        LoginSteps.java
        RegistrationSteps.java
      hooks/
        Hooks.java