Well.....here's the deal. The metric sizes were SUPPOSED to bring standardization to the tire industry. I remember well the pitches from the tire companies how they would all be the same size from one manufacturer to another. Standardized. It did anything BUT. No two tire maker's same sizes ARE the same size....and I'll just comment on the P-metric tire sizes...
The plus is they give is an actual width, albeit in metric format.
That makes it easier to calculate sidewall height as well.
Plus, then you know that all say- 235 width tires are the same width (or are supposed to be).