Putting it another way, you can make libraries out of a program with this technique.
Unless you meant copying source code from GitHub, which isn't an option if the program is a 1998 PlayStation video game and you don't have the source code for it.
You can do things like decompiling a program piece by piece like the Ship of Theseus. The linker will mend both original and rewritten parts together into a working executable at each step.
If you change the functionality of the rewritten parts, you have a modding framework that's far more malleable than what traditional binary patching allows.
As for quick one-shot jobs, I've created an assert extractor for a PlayStation video game by stuffing its archive code inside a Linux MIPS executable and calling into it, without figuring out the proprietary archive format or how the code inside the delinked object file actually works.
* (excluding languages that compile to an IL like C# of course, but decompiling C# is trivial)
It turns out you can. Linkers do not care about types or data structures, all they do is lay out sections (a fancy name for arrays of bytes) inside a virtual address space and fix up relocations.
I've written case studies on my blog where I've successfully carved out whole pieces out of a program and reuse them without reverse-engineering them. I've even made a native port of a ~100 KiB Linux a.out x86 proprietary program from 1995 to Windows and all I had to do was write 50 lines of C to thunk between the glibc 1.xx-flavored object file and the MinGW runtime.
One user of my tooling managed to delink a 7 MiB executable for a 2009 commercial Windows video game in six weeks (including the time required to fix all the bugs in my alpha-quality COFF exporter and x86 analyzer), leaving out the C standard library. They then relinked it at a different base address and the new executable works so well it's functionally indistinguishable from the original one. They didn't reverse-engineer the thousands of functions or the hundreds of kilobytes of data inside that program to do so.
This is complete heresy according to conventional computer sciences, which is why you can't apply it here. I'd be happy to talk at length about it, but without literature on the topic I'm forced to explain this from basic principles every time and Hacker News isn't the place to write a whole thesis.
If I were to guess, you're saying that you reverse engineer the API boundary without reverse engineering the implementation. But then figuring out what the API contact is without documentation seems intractable for most API boundaries.
