It means the rim manufacturer wants to be precise as to rim stats. It makes it a little easier to know what size tire is
optimal, which referring the doubly numbered rims, the tire sizes the rim maker feels the rim is suited for.
I disagree WRT the example given and the common experience of tires of vastly different widths being OEM speced on rims with this "1.5" label. (Note I said "label" not dimension or size). If fact it could be said that having a tire reference on a rim is misleading.
Back in the beginning of the MtB movement we (the USA) had various import tariffs on bikes and parts (IIRC it was the industry group known as BMA/6 that lobbied and attained said tariffs). These tariffs used the dimensional language that was in use by the US manufactures at that time. So 26x1.75, 26x1 3/4, 26x2.125 were all the spec that triggered a tariff. So the foreign manufactures simply changed their labeling of some parts to end up with a work around of the tariffs. These different labels caught on by the buying public (if for no other reason then the need to believe their non US made bike was better and unique then the gas pipe stuff the US mostly made).
Is this what really happened? I have never had a manufacturer industry insider or a government employee confirm this but I have heard this story from a number of supply side and retail sources over the years. Andy.