The working principle of APPEND is not complicated. It primarily serves as a bridge between old DOS applications which have no or poor support for directories, and users who really, really want to organize files and programs in multiple directories and possibly across multiple drive letters. Of course the actual APPEND implementation is anything but straightforward.
↫ Michal Necasek
Another gem of an article by Michal Necasek, detailing a command I’ve known about almost all my life but never once knew what it was supposed to be for. The gist is that APPEND allows for files to be opened not only in the current working directory, but also up to two levels deeper. This gives you a rudimentary way of working with directories, even when using programs or commands that have no clue what directories even are. since DOS 1.x doesn’t support directories, but DOS 2.x does, having a tool like this to create a bridge between the pre and post-directory worlds can be quite useful.
I’ve basically learned more about DOS from Necasek’s work in the past few years than I learned about DOS when I was actively using it in the early ’90s.