Hey Kundan, as of now selenium is gaining momentum because it has many advantage compare to other paid tools like HP UTF /QTP .The main reason selenium is going to be used a lot in next five year is its a free tool and has a huge community which provide many solution .Until unless any other tool with more feature and customization come selenium is going to be the main tool used for UI testing and mobile application testing .
Selenium is considered as one of the best opensource test automation framework, which helps to prepare & execute test cases with automation. This is the most preferred test automation framework by software testers as well as QAs. Selenium has the support of some of the largest browser vendors who have taken or are taking steps to make Selenium a native part of their browser. Hence, Selenium framework is widely used in developing test automation tools, which is further used for UI testing.