There are many different advantages to using a string resource file, rather than hardcoding strings. While it may seem pointless and convoluted in small projects, it is incredibly useful in larger projects.
If you know that you want the same string to appear in multiple places, possible occurring in separate activities, the string resource file allows you to reference one source, rather than type the string over and over again.
If you decide to change a string in you app, or have a dynamically changing string, it is much easier to modify the resource file than it is to manually change all instances of that string in you app
It is great for localization and translation. If someone wants to port your app to a different language, all you have to do is substitute the
strings.xml with a translated version
Thus, it may seem useless in small applications, but for large scale applications, it makes your life much simpler and allows you to more easily reach foreign language users.