Unix is the most common and portable platform used in the IT Industry. You do not need to be a Unix developer to be working on the Unix platform.
Even application developers and software testers have a regular business requirement to work on Unix environment. Whether as a developer you are checking the log files or deploying application, you are doing some backend testing, basic knowledge of UNIX always proves to be very useful and handy.
In fact no interview for a developer/ manager or tester position is said to be complete if there are no questions asked on UNIX. This app comes loaded with commonly asked questions on UNIX.