To whom? To other branches of physics? Look up AdS/CFT. To the general public? Dunno, I guess the pursuit of understanding the universe is its own reward for some.
You mean, if string theory does not turn out to describe the universe, how could it be useful? Well, by giving extraordinarily powerful tools for understanding things that are well established to be useful, like quantum field theory. AdS/CFT gives us the only tool we have to analytically understand quantum field theories in the so-called “strong coupling regime”. This is useful for discovering new properties of quantum matter in systems where you would otherwise need simulations. You can think of it intuitively as string theory providing a glue between two descriptions of the quantum matter, like a “type cast” in programming where you start with one kind of object but reinterpret it as another. The thing that is incomprehensible in one representation is simple in the other. This was discovered by studying limits of string theory in interesting geometries.