/etc/mailname

Why do people make files that have one line of text? Why not just include it in the config file? Do lots of programs use /etc/mailname? what for? what purpose? What is the point?

Ah well. My return-path being wrong was corrected by editing myorigin, which was actually set to /etc/mailname. For whatever reason. Goofy computers. I hope no one lost mail because of that...