Ruby
 

What is this language good for? Like C is good for system programming, java for web and Fortran for math, what is this language good for?

Yep. General purpose programming language. But that said, its mainly used for web programming.

Mainly ruby is used for web programming with ruby on rails
But ruby can be used as stand-alone software too. (you can see on rubyforge or github)

Shrug ... "What's a hammer good for?"

It's a tool. All of them are tools. Different language-designers at different times decided that the world really needed a new language ... so they designed one and implemented it. Perhaps the language then disappeared into the bowels of obscurity. Or, perhaps, Tim O'Reilly published a book about it with some sort of animal on the front cover. ;)

Each language was designed, more or less, to "facilitate" something ... to make "something" '–er' to do ... and enough people decided for whatever reason to use the language, such that it "caught on." People decided that it was a "useful" tool, and decided to start using it to do the stuff that they get paid-for.

It is definitely an interesting experience to design and implement a programming language . . . and, really, not as hard as you might think.