Why C is on the top of programming language?

C is on tiobe(Base on http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html) the top of programming language of choice? What are the well known application today that is written in C that makes it on the top? I'm programing in PHP and Javascript a years now. I decide to go back to C and to study it because I felt my skills are not enough and sometimes it is in the requirements of the employer. and I want to be comfortable in different language to be flexible in my future job. Currently I'm doing some basic console programs. What are the benefits of learning this language for the future? What are the applications that are now on the market that is written in C? Thanks.