Mine has a scale marked in 0.02mm steps. The top does internal diameters, the bottom does external diameters, the end is a depth guage. It's here..
Essentially, the vernier shows me if parts I've assumed to be a certain size, really are that size, and therefore if they really will fit with other parts properly. So useful when pairing up handlebars with stems (25.4mm vs 26mm especially). Ditto seatposts and seat tubes (27mm vs 27.2mm). I used to squint at a ruler for this, which, when I think back, was obviously daft, as a ruler was not nearly accurate enough.
And yet if someone had told me I should buy one ten years ago, I'd have said, no thanks, I don't need one.
So I guess sometimes you don't know how useful a tool is until you own one, and you have it to hand. Perhaps that's a special category of tools: the ones you don't know you need until you've got them, and then you can't work without them. Which other tools come into this category?..
