Hi Daragh,
Selenium is very used especially because many testing activities are related to UI testing on web and mobile applications. Automation is continuously growing and a tester with programming skills finds Selenium perfect for developing automated tests; testers can choose almost all programming languages they want and they can use advanced IDEs for writing code. It is also preferred because it is free and there are companies that cannot afford purchasing expensive tools.
An interesting side is that paid tools are now supporting also Selenium; this is a clear evidence that it is very important for testing.
So, for UI testing we can say that Selenium is most probably the best tool.
But the topic regarding tools is a bit larger because we have to take into consideration other testing activities like testing desktop applications (where Selenium is not an option) or API testing (where tools like SoapUI or JMeter are very good and largely used).
Regards,
Alin