Remember when you were a kid splashing through all the puddles and kicking up mud in a good thunderstorm. Never thought you'd have a good excuse to do it again? Well, buying a home is your reason now. Why is it as great of an idea to look at homes during or just after a good rain as it is to see it in the daylight hours? Ugliness gets exposed. Viewing a home when it has recently rained gives you a great chance to examine any potential problems that you may not see on a perfectly sunny day. Does the yard get flooded, how's the basement hold up?? You can't tell how light will come into the home at night just as you can't tell how damp the basement will get during a storm if it's dry. This will allow you to spot any leaks in the roof or basement immediatley.
Another advantage is the same advantage that looking in cold or snowy weather has, less competition. In bad weather people do not like to go out to search for homes so there is an advantage to viewing a home during this time. Let's say it rains for three straight days, you view homes, write an offer, and get your offer accepted by the time the sun comes out for the rest of the crowd. You just beat your competition to the punch. Now they are the ones who lose out on choice, and meanwhile you have found your dream home.
So remember, even though you're not a kid anymore it's still fun to play in the rain.