I just like the idea that it could finally tell us if "earth sized" planets are the norm or the exception. It will also be able to tell the distance from the sun. How many of those same planets are in the life as we know it habitual zone.
It will I am sure also be able to contribute a lot of information on typical planetary systems, and how they are structured. I know we have found a few Mega Earths (that is to say Large Rocky planetary bodies), some super Jupiters and so on.
I don't believe it is able to detect atmospheric composition (ie Hydrogen, Oxygen, Nitrogen...) but I don't know for sure.