Anyone reading this site knows how difficult it would be to find data that a person has taken even a little bit of effort to hide, particularly on a laptop (which typically has more storage than a phone or tablet, not to mention complete access to the file system). All the laptops, phones, tablets, MP3 players (if anyone still uses them), digital cameras, IoT devices, etc., of all the people coming through all the checkpoints at all the airports with flights going to the US in just one day would be more than an army of analysts could check in a year, unless they think that terrorists typically leave files named "terrorist attack info.doc" sitting on a Windows desktop or the equivalent. It's not just a needle in a haystack... it's an object that may or may not be a needle (if it even exists) in any one of millions of haystacks scattered across tens of thousands of farms somewhere in the world.
If there is data secreted on a hard drive, a determined analyst may well be able to detect its presence, or at least suspect that seemingly random data somewhere might actually be hidden encrypted data, but finding such things isn't something that can be done within the context of a check at an airport with every device that comes through (with nearly every person in the line having at least one device, and possibly several).
It seems like a more likely plan is to provide cover for those few incidents where a traveler is a known person of interest and the government agents want to have a chance to image the device's storage and have an excuse ready, like "you were randomly chosen" or "we do this for everyone." Or, even more cynically, it could just be part of an ongoing effort to condition people to accept the ever-increasing intrusion into their lives in the burgeoning Stasi-style surveillance state that the US (and probably every other industrialized country) is fast becoming. Your papers, please!