Hotel rooms have fairly low security standards by anyone's reckoning. Most hotels won't even guarantee your in room safe's security (and if you don't believe me, read the small print you sign when checking in)!
On top of nearly unrestricted access when you're out of the room from maids, managers, maintenance, etc, it's also pretty easy to get into your room when you're there. Yes, you can deadbolt some room doors, which means only (hopefully, not sure how this works) authorized technical maintenance should be able to open it. But that's far from all hotel room door locks, and many can be opened with this video's techniques, relying on some cleverly bent metal wire and string:
This post has a video, click through to the site to view it if you can't see it below.
As you can see, just jamming a towel into the handle will make this technique much more difficult. So, if you're in a room you can't deadbolt, this is a good second way to 'hack-safe' the door handle. And even if you can use the deadbolt, this won't take more than 15 seconds and is a good second line of defense.
Definitely worth doing if you're in a sketchy hotel (like I usually am!) or are concerned in other ways.
You should follow me on twitter here.