How does a keyfile work?

I was looking at some code on codereview that is far out of my league of understanding and just screwing around, but it raised a question.

This program (along with many other encryption programs, i assume) uses a keyfile.

Now, if I use /dev/random as my keyfile, it'll (theoretically) never be the same. Therefore, I could never decrypt the output file.

Is this feature unique to One Time Pad encryption tools? How am I supposed to decrypt it?

What do other tools do differently to allow me to decrypt it in the future?