Ne comprenez pas pourquoi $ _Session est nécessaire

J'ai essayé de créer un formulaire de connexion à partir d'un exemple. J'ai fait ça marche mais je ne comprends pas.

pourquoi $ _ session ['umsco'] code> est requis et pourquoi dois-je attribuer la variable code> $ (code> $). Je ne comprends pas non plus si conn-> ferme () code> requis et quel est le résultat $ code> variable. P>

// I include the database file so we can get the information from there and apply it in here as well.
include "dbc.php";

// here we select the variables we want from the table 'members'.
$sql = "SELECT id, firstName, lastName FROM members";

// here we do something that I dont really know but this is standard procedure.
$result = mysqli_query($conn, $sql);

// declaring username
$username = $_POST["username"];
$password = $_POST["password"];


// my result thing, again, standard procedure.
$result = $conn->query($sql);

// we insert all values in row, standard procedure.
$row = $result->fetch_assoc();

// here we check if the username is the same as the database.
if ($username == $row["firstName"]) {
    // if it's correct then we start a session
    session_start();
    // we give a session some random letters or numbers and set it to $username, not exactly sure why, but it wont work without it.
    $_SESSION['umsco'] = $username; 
    // we change the location to secrect.php
    header("location: secret.php");
}

// we close the connection, not sure if this is needed, but it seems logical.
$conn->close();