Desired capabilities are a way to set certain properties for a WebDriver. They are used to specify the desired properties of the browser that will be launched when a test is run. These properties include things like the browser name and version, the platform the test is being run on, and the proxy settings to use.  One of the main reasons desired capabilities are necessary for Selenium is that they allow you to specify the exact browser and platform you want to use for your test. This is important because different browsers and platforms can behave differently, and you may want to test your application on a specific combination of browser and platform to ensure that it works correctly.  For example, let's say you are testing a web application that you know works correctly on Google Chrome on Windows. You might want to use desired capabilities to specify that your test should run on Chrome on Windows to ensure that the test results are accurate.  In addition to specifying the browser and platform, desired capabilities can also be used to set other properties such as the proxy settings to use, the JavaScript settings, and even the browser's user agent string.  Overall, desired capabilities are an important part of Selenium as they allow you to customize the browser and platform that your tests run on, ensuring that your tests are accurate and relevant.