If your lifetime plan includes writing an article or a book, you may need to learn how write a rather large document in LaTeX. Or maybe you are co-writing a document (Andrew and I did a big homework once and synchronizing files can be tricky).
The subfiles package can help with that. The documentation for subfiles is here.
It allows you to write a master file and several subfiles that can be compiled independently, so you can work on and compile each subfile (for example, a chapter or a section) and see the results, and compile the master file to see the full book. A simple and useful example can be found in Wikibooks.