Hello, Zap. I think the conventional wisdom behind the practice of not getting the tomato plant too wet is that a wet plant encourages fungal and bacterial breeding grounds. The tomato is succeptible to a wide variety of pathogens that could be present even in healthy soil. Their major point of entry to the plant is through the leaves. Watering from above encourages the soil containing these pathogens to splash up on the plant and leaves, and from there absorbed throughout the plant.
I think a lot of gardeners assume that if something bad can happen, it will, so they choose the 'ounce of prevention is worth a pound of cure' truth and choose to water the plants in a way to prevent the whole issue.
Of course, we can't do much about the rain
it does the same thing. But plants held up away from the soil with cages and stakes, and trimming some of the lower leaves help with what we can't control.