On modern CPUs that actually can't run at full speed for thermal reasons, they're critically important (though a complicated dance with MWAIT and a ton of drive

On modern CPUs that actually can't run at full speed for thermal reasons, they're critically important (though a complicated dance with MWAIT and a ton of drivers has supplanted HLT on x86 devices). On microprocessors of the time, they're indeed a little useless. None of the logic was going to disable the internal clock, this was decades before the introduction of gateable power wells, etc... But on the bigger hardware where DMA was common, a halted CPU could be relied on not to be issuing needless requests to the memory bus and other clients like I/O devices (SMP was in its infancy in the 70's too) would have lower contention and higher throughput. I'm sure that was part of the thinking. The IBM PC itself tended not to contend on its bus much (CGA and MDA had their own framebuffers and floppy DMA was mostly a joke), but maybe there were other 8086 implementations that cared.