The current trend is toward wider rims, and by extension wider tires. Whereas ten years ago, road bikes were equipped with 19-23 mm tires, most today are mounted with 25 mm or even 28 mm. This reduces rolling resistance whilst increasing comfort and control.
For those who appreciate the finer aspects of aerodynamics, a rim width that matches a specific tire width provides a smoother transition between the two components. A tire and rim combo that features a flatter transition helps reduce drag as the wheel cuts through the air.
Numerically speaking, most CORIMA wheels have an outer rim width of 26mm, ideal for tire or tubular widths of 25mm.