I've never heard of that either. Surely useful to a manufacturer of tubes. I could see it being useful in that it might give you a real measurement of how much rubber you are getting. As you may know, bike tires and tubes are measured with nominal numbers that don't actually reflect the real size of the product accurately. So you might be able to tell how wide a tube is without the nominal numbering. Or, if the nominal numbering is accurate.
For example, I've noticed the more recent batches of innertubes we've had at our store are narrower for the same nominal size. This would mean a lighter, thinner inner tube in use (Any size tube will fill the same size space of the tire if possible, less rubber means it has to stretch more to fill the same space). And I've noticed a slightly higher rate of defective tubes. But I haven't actually measured anything, this is just my impression over a period of time. It's hasn't been an important issue.
Inner tubes in bicycles are sized quite approximately and one can usually fit the wrong size tube into a tire to make if work if you have only got that one option. I'm someone who would recommend a thicker tube, as I favour that reliability over the small weight gains. So I would use that information to pick wider, thicker tubes. Others might do the opposite and choose narrow for the weight savings. All of this is assuming consistent quality, which is a big if.
Anyway, I agree with you there is little need for this number to most people. I'm just expanding on the idea to show there could be some use for a very few. Mostly people who overthink trivial matters ;-)