My impression: many of them cannot afford to keep a house in their home district and another place of residence in our Nation's capital. DC is not a cheap date, rent or mortgage wise.
Some of them literally sleep in their offices. Many of them rent rooms in tounhouses and the like, as many of us did as we were getting started as adults and eventually moved on from such an experience. (I am picturing a "Friends" sitcom, where the folks in the show are elected officials, not the people that were actually in the program when it was on the air/cable.)
I can't help but wonder how this affects how they vote on things before them.