You need this file for I/O (Input/Output). If you care to, you can look at it... it should be in /usr/include. Explaining the specifics might not be that helpful if you haven't used your own .h files yet. At a very high level, these files are used to define function prototypes (since you haven't used functions, what a function prototype is won't really make sense), constants using #define (don't think you've covered this yet, either), typedefs (probably not yet), structs (don't think you've seen them), and assorted other pieces of data you'll need to make use of a standard library. Basically, someone else wrote some code. You may or may not have access to the source code, but you do have a compiled library. They have provided you with a header file that will act as a guide for what functions are available (to help both you and the compiler) and what datatypes will be used when interacting with the library.
A lot of programs make use of file and console I/O, so a lot of programs will use this file. I don't think anyone could say a percentage, but it's much closer to 100% than 0% or 50%.
-Lee