[kens]

Yes, halt is sort of redundant and processors like the 6502 omitted it. I think the historical popularity of halt was because you could indicate to the operator that the computer was halted, rather than in an infinite loop. Peripheral devices could also detect the halt state.

[flohofwoe]

On Z80 home computers, HALT was often used to synchronise some piece of code with a hardware interrupt like VSYNC for 'race-the-beam' stuff (because an interrupt takes the CPU out of the HALT state).