Do I need to run selenium server when using chromedriver?

Question

I'm new to Selenium and Nightwatch and installed selenium-webdriver first, then chromedriver and finally I installed Nightwatch (because the syntax looks nice to me). Now when I want to run a single test with nightwatch, I always get this result:

> ./node_modules/nightwatch/bin/nightwatch -t nightwatch-node-scripts/test.js 

[Test] Test Suite
=====================

Running:  Demo test Google
Error processing the server response: 
 unknown command: wd/hub/session

Error retrieving a new session from the selenium server

Connection refused! Is selenium server started?
{ value: -1, error: 'Unexpected token u in JSON at position 0' }

The chromedriver is running and listening on port 9515:

> lsof -Pnl +M -i4 | grep 9515
chromedri  6489     1000    8u  IPv4 867952      0t0  TCP 127.0.0.1:9515 (LISTEN)

which is the one I use in my pretty basic nightwatch.json:

{
    "src_folders": ["tests"],
    "output_folder": "reports",
    "custom_commands_path": "",
    "custom_assertions_path": "",
    "page_objects_path": "",
    "globals_path": "",

    "selenium": {
        "start_process": false,
        "server_path": "",
        "log_path": "",
        "port": 4444,
        "cli_args": {
            "webdriver.chrome.driver": "/usr/local/bin/chromedriver",
            "webdriver.gecko.driver": "",
            "webdriver.edge.driver": ""
        }
    },

    "test_settings": {
        "default": {
            "launch_url": "http://localhost",
            "selenium_port": 9515,
            "selenium_host": "localhost",
            "silent": true,
            "screenshots": {
                "enabled": false,
                "path": ""
            },
            "desiredCapabilities": {
                "browserName": "chrome",
                "chromeOptions" : {
                    "args" : ["--no-sandbox"]
                },
                "acceptsCerts" : true
            }
        },

        "chrome": {
            "desiredCapabilities": {
                "browserName": "chrome"
            }
        },
    }
}

Answer 1

From Selenium Documentation:

You may, or may not, need the Selenium Server, depending on how you intend to use Selenium-WebDriver. If your browser and tests will all run on the same machine, and your tests only use the WebDriver API, then you do not need to run the Selenium-Server; WebDriver will run the browser directly.

There are some reasons though to use the Selenium-Server with Selenium-WebDriver.

You are using Selenium-Grid to distribute your tests over multiple machines or virtual machines (VMs). You want to connect to a remote machine that has a particular browser version that is not on your current machine. You are not using the Java bindings (i.e. Python, C#, or Ruby) and would like to use HtmlUnit Driver