GNU was started in 1984 by Richard Stallman. At that time Richard Stallman worked at MIT in the AI Lab. Certain politics played out at the time, and Stallman found himself twisted in the middle of a tangle of proprietary software that did not lend itsself well to research. Stallman resigned the lab and vowed to one day develop a complete operating system that one could freely give to his friends. The answer is, there are thousands of applications for performing tasks from recording CDR's to plotting graphs.
Software is available for even the most imagineable tasks.
Certain credit is due to BSD too, the Berkeley Software Distribution. With the Linux kernel, GNU software, BSD software, we have a complete operating system that we can create solutions, work, and produce with. And when we make something useful, we can sell it to our friends, who in turn can add more useful features to it. If it's GNU software we sell our friends, we have to give them the source, and allow the buyer the right to modify the code. The reason I used the example of selling our software is that GNU is about freedom, not necessarily about software being free.