Right now, there are six people living and working in the International Space Station as it hurtles around our little oasis among the stars. That thought alone is astonishing/freaky enough until you realize the sheer number of things that could go wrong (Gravity, anyone?). That's why it's no surprise that NASA wants the new ships that'll ferry people and supplies to the ISS to act as lifeboats in case things up there take a turn for the worse.
As you might expect, a spacecraft has to meet some very specific criteria in order to count as a space lifeboat. It's got to be able to bring four people back home, sure, but it also needs to be able to house those astronauts for 24 hours should they need to seek temporary refuge. Oh, and it has to withstand micrometeoroid impacts and be reliable enough to work properly after over 200 days of neglect... while sipping only enough power to run your refrigerator. NASA's need for new lifeboats doesn't just have to do with getting astronauts home in a hurry, either. The ability to get more people off the station means it can safely host more people in the first place, so there's the potential for much more envelope-pushing orbital science to happen.