I don't really understand where the question comes from?
JUnit is a
unit testing framework for
Java. It is open source and free to use.
It's a .org domain name, so in general they must be a non-profit organization. They may sell some documentation or shirts or something like that to cover costs.
Not sure what kind of "reasons" you are looking for... reasons for existance are more for the philosophers. :-)
Does this help?