Originally Posted by Carbonfiberboy

I'm not sure the correct word is "want." Until very recently, standard road rims ran from 15mm-19mm outside. Tires are still designed for that geometry because that's what most folks still run. Of course you can run a 25mm tire on a 25mm outside measure rim. The question is what's optimum. For sure on a deep aero rim designed for TT, 25-25 would be good. As above, some rims are designed for 23-25. However as also noted above, going with a wider rim exposes portions of the tire to the road surface which were never designed for that except when cornering.

Although many on here advocate lower pressures for use with wider rims, I've found that I get more flats and sidewall cuts if I do that. I've started inflating my tires on wide rims to greater pressures than I used on narrower rims, which fixes the sidewall=on-the-road issue. I still get the benefits of greater rim strength and more aero cross section of the wider rims.

As usual with engineering, there's not one best answer.