There are a lot of utility programs for text manipulation, scanning and so on. Often they are written as filters so you can use them with shell redirection like this.
dosomething < infile.txt > outfile.txt
There's nothing wrong with this as such, but the problem is that this causes problems when you invoke the program from somewhere else than a unix shell prompt. As an example if you need to invoke it from a compiled binary, things get very complex as you need to juggle with pipes and other stuff.
Because of this you should always make it possible to specify inputs and outputs as plain files. For new programs this is simple. This is a bigger problem for existing programs that already have hundreds of lines of code that read and write directly to stdout and stdin. Refactoring it to use files might be a lot of work for little visible gain.
No matter, the C standard library has you covered. It has a method called freopen that opens a file and replaces an existing file descriptor with it. To forward stdout and stdin to files you just need to do this at the beginning of your program:
freopen(ifilename, "r", stdin);
freopen(ofilename, "w", stdout);