Why do we need Session Handling?

During test execution, the Selenium WebDriver has to interact with the browser all the time to execute given commands. At the time of execution, it is also possible that, before current execution completes, someone else starts execution of another script, in the same machine and in the same type of browser.
In such situation, we need a mechanism by which our two different executions should not overlap with each other. This can be achieved using Session Handling in Selenium.