Perhaps not a hard requirement, but a handy feature. It would be nice to know that "<number> <street>" is at least in the area of "<a number close to that number> <same street>" (if the city doesn't completely mess up giving out addresses).

With what3words, "///ladders.chimp.drama" is exactly next to "///thick.candle.enacts" and "///nails.spit.blaring", but you can't tell. Also, you don't know whether there are other addressable points in between. It would have been nice if there was more method in this madness.