Web browsers are (quite literally) the defining feature of the World Wide Web, which was invented when Tim Berners-Lee released the first version of his HTML browser (World Wide Web) on Christmas day 1989. In other words, they are what makes the web the web.
For a variety of historical reasons, users tend to treat web browsers as utility-grade software—a part of the operating system they expect our devices to have already installed rather than a piece of software you choose to install and run. But more than one kind of browser exists and there are differences between them. Sometimes one browser is better than another for certain tasks or sites. You should know what browser you are using and you should make sure you have some alternates installed.