It's very useful if you don't have access to the original source code.
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.
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.