What is the use of these hex codes and why people use it?

Hi, I've been working on binary data and buffers. And came here to check the example of .zip listings. And looked at the code but I was wondering why there are so many hex codes. What are their usages what value do they return and how are finding out the right hex code to compare. 
here is just a one file.
https://github.com/davidflanagan/BufferView/blob/master/BufferView.js#L300