Is it really that irrelevant? I mean, if you look past the specifics (directories, interrupts, DOS versions), this seems to be implementing the idea of bringing something into scope, in particular bringing it into scope from the outside, to modify the behavior of the consumer (here, assembler) without modifying the consumer itself. Today, we'd do the equivalent with `ln -sr ../../inc ../inc`.

I'd argue the general idea remains very important and useful today, though it's definitely not obvious looking back from the future what this was what APPEND was going for.