Most digital data is stored in binary files. Pure text files are somewhat rare (probably less than 2% of the data in the world). There are several reasons why binary files are used.
I. Input and output are much faster using binary data. Converting a 32-bit integer to characters takes time. Not a great deal of time, but if a file (such as an image file) contains millions of numbers the accumulated conversion time is significant. Computer games would slow to a crawl if their data were stored in character form.
II. A binary file is usually very much smaller than a text file that contains an equivalent amount of data. For image, video, and audio data this is important. Small files save storage space, can be transmitted faster, and are processed faster. IO with smaller files is faster, too, since there are fewer bytes to move.
III. Some kinds of data can't easily be represented as characters. For example, the bytecodes of a Java class file or the machine language of an executable file. You may not usually think of this as data, but of course, it is. The Java compiler reads an input file (a source file) and writes a binary data file containing its results (the bytecode file).