You're riding along and see the increasing darkness ahead of you. It gets darker and darker. You see cars approaching you with their lights on and then you see wipers going. The wind picks up. You begin looking for a temporary shelter where you can change into your rain gear. You finally stop and get as far off the road as you can in a sheltered place. You get your rainsuit from your saddlebags or trunk and fight it over your jacket, pants, and boots. Hopefully you have some special rain gloves or even a pair of yellow dishwashing gloves. Some even have rubber rain boots to slip on over their boots. Of course, you also have a fullfaced helmet to encase your head and keep driving rain out of your eyes. You are ready for the rain and cautiously ride off. Of course, an extremely heavy rain with puddling, lightning, and thunder should cause you to give up on your riding and seek some semi-permanent shelter to wait out the passing storm.
On the other hand, some riders ponder over better ways to keep dry in the rain. Here are some pictures and a video from core77.com that document one man's attempt to stay dry. You may or may not agree with his approach or want to buy his product. Take a look.