Proficient Motorcycling and braking in rain

I read David Hough's Proficient Motorcycling annually in preparation for the start of riding season. There is one thing in his book that I just don't understand. On page 72 of the second edition, he says, "In the rain or when carrying a passenger, more rear brake can be applied because there will be comparatively more weight on the rear wheel." The part about carrying a passenger makes sense, but I don't understand why more rear brake can be applied in the rain when riding solo. When riding solo there is no more weight on the rear wheel and I don't understand how rain makes a difference in this.

Comments appreciated.

In the rain you will not be as aggressive with the front brake so there will be less weight transfer to the front so as a result you will have more weight on the rear for the rear brake as compared to dry pavement when you can be aggressive with the front and transfer a lot of weight to the front wheel.