How do we use transits today?
When you look at the planet Venus passing in front of the sun, realize that you have a front row seat to the same phenomenon that the Kepler spacecraft is trying to witness around stars that are light years away. Realize also (especially if you have the misfortune of being on the night side of the earth opposite the direction of Venus and the sun during the transit) that somewhere out in distant space someone may be looking toward our star and could detect two planets--Venus and Earth--passing in front of the sun.
Scientists will also be studying Venus during the 2012 transit of Venus to understand its atmosphere, which swirls rapidly around the planet. See http://science.nasa.gov/science-news/science-at-nasa/2012/04jun_arcofvenus/.