Explain the high-level reasons that are responsible for development of selenium tools from Selenium WebDriver to Selenium Grid?

Prima facie, if you look out at Selenium WebDriver it seems to be a complete tool in itself. It supported various programming language, Cross browsers, various OS and has better and improved performance when compared with previous tools under Selenium Suite. However, there were few limitations to it which resulted in development of Selenium Grid. Selenium WebDriver couldn’t run parallel tests across different machines and different browsers all at the same time. Parallel execution means running multiple tests at once. In order to cater to this requirement, Selenium grid was introduced.