Having a back up of important files on a USB drive can be helpful in or after a survival situation.
In the event that everything you own is destroyed except what in with you in your bug out gear carrying some important information can come in handy. If there is a terrorist attack how does the government know you are who you say. How does anyone know you are not a terrorist. You were of course prepared ( we always are). Without your documents you do not exist. Though having a back up is not as good as the original it is still better than nothing. Here is a list of things you may want to keep on your USB drive. I would also recommend having a paper copy at home in a fireproof water proof container.
Social security card
Bank account information
In some situations a USB will be useless but hardly take up any room and could benefit if needed.
Keeping all this information in one place is risky. If someone where to get a hold of your flash drive someone can use the information against you or to steal your identity. Password protecting or encrypting files might be a good idea.