When scientists want to understand something they will often use an experimental “unit” and a control “unit”. And when studying something if you have a second copy around then you can control for more kinds of factors. And when it comes to people identical twins are the closest you can get to a second copy.
Genetically they are the same, so you can rule out genetic differences. Though identical twins can still be very different. This is one way that scientists can determine when something has a genetic basis rather than some other. Nature versus nurture. Combine that with the overall similarity of identical twins and you have a nearly perfect way of comparing effects.
So when Scott Kelly went up to the International Space Station at the end of March, NASA was lucky enough (or far sighted enough) to pick someone with an identical twin. In fact Scott's brother Mark was an astronaut flying on four Space Shuttle missions before he retired. (Mark Kelly is also the husband of Gabrielle Giffords, the congresswoman that survived the assassination attempt back in 2011.)
So Scott Kelly is going to be up on the International Space Station for a year. And he and his brother will be examined to see what affect the long stay in space has on his body. There are know issues such as bone loss. But they are going to look at psychological effects, immunological effects, how long periods in space affect vision, and many other ways that space could impact the human body. Taking strong advantage of the twin as a control.
One of the benefits of such a study on the long term impact of space travel on a human body is in planning for trips to Mars. Even with no restriction on the amount of fuel, it takes hundreds of days to get from the Earth to Mars. And all that time in space will affect the astronauts making the trip. So knowing ahead of time what can and will happen to the astronauts is essential in forming a reasonable plan for getting people to Mars. So the opportunity to have a twin study for these effects is far too good to pass up.
Sadly one thing they won't be able to control for is diet. Scott is going to be limited to “space” food, while Mark will have a normal earthbound diet. That alone could be a significant psychological impact. I know I'd have trouble with that.