[cesaref]

I think the history of this is that these operations were common with assembly programmers, so when C came along, these were included in the language to allow these developers to feel they weren't leaving lots of performance behind.

Look at the addressing modes for the PDP-11 in https://en.wikipedia.org/wiki/PDP-11_architecture and you'll see you can write (R0)+ to read the contents of the location pointed to by R0, and then increment R0 afterwards (so a post increment).

Back in the day, compilers were simple and optimisations weren't that common, so folding two statements into one and working out that there were no dependencies would have been tough with single pass compilers.

You could argue that without such instructions, C wouldn't have been embraced quite so enthusiastically for systems programming, and the world would have looked rather different.

[monocasa]

Additionally, those indirect memory instructions ended up disappearing because it complicated virtual memory implementations. It was a pain in the ass to describe the multiple places in memory an instruction could be accessing and which actually faulted to a fault handler, not to mention having to roll back all that state on more complex designs.

[IshKebab]

I worked on a more recent custom AI ISA that had that too. Pretty neat; I'm surprised it's not more common. I guess it doesn't matter so much now that memory is so much slower than ALU ops.