Why connect and close mongodb client after each query?

Question

I'm learning mongodb to create a API, and on every exemple or tutorial I see about using mongodb on Node.JS with Express the logic starts with a client.connect, the query is done and client.close() is called. Is it really necessary to connect before every query ? Isn't it possible to make a connection for exemple when a user login ( first query ) , and then as i'm going to do lots of query's while the user is logged i don't need to establish a connect after all of them, if the user logoff, thats when i want to close the connection.

Another question, is there another type of call i can do to replace .toArray(function (err, result) {} ? It's returning all my data inside a array with a single element and i need to do result[0] sometimes, which seems awkward.

const search = {
    multiple: function (query, projection, callback) {
        console.log('multiple was called');
        client.connect(url, function (err, client ) {
            if (err) throw err;
            console.log('Connected to db');
            const db = client.db(currentdb);
            const collection = db.collection(currentcollection);
            collection.find(query).project(projection)
                .toArray(function (err, result) {
                    console.log('Result on the way');
                    if (err) throw err;
                    callback(result)
                    client.close()
                });
        });
    }
};