What’s the use case for running a 32bit binary in a 64bit cpu/os? Is there any advantage? Or is it simply to avoid having to recompile twice to support two architectures?
First advantage is that the pointers become tiny, so the objects took up a lot less space. The other advantage was that when it was time to serialize and deserialize the data, you didn't have to do any processing at all.
Just reinvented segmentation
The next generations will not include any core supporting the 32-bit ISA.
If you have a statically compiled binary all you need is compiler support for 32-bit address mode in the otherwise 64-bit ready instruction set and on x86 you get access to more registers etc but you don't pay the price for the wider addresses stored in data structures.
OTOH if you use shared libraries you also have to have your shared libraries compiled in that mode too. The difficulty of dealing with that depends also if you gave a hybrid system or a full 32 bit userspace
Just a side-note, I am always intrigued by 16bit thumb and aarch32 "interworking", basically flipping on/off the LSB of the PC(EIP/RIP equivalent register) to tell the cpu that the next instruction is thumb mode or a32, letting them mix different instruction sets within the same program.
Granted this might not actually be worth considering here, given the much shorter history unlike x86(-64).
Notibly, conditional execution is gone.
If you have a compiled binary but lack source, then this subsystem is helpful.