DirecTV receivers have one or more USB ports on them, but most USB accessories won't work with them. This might make them seem vestigial, but the USB ports on DirecTV receivers do have a few uses, like connecting an over the air antenna module or providing power to USB devices.
You can purchase an AM21 Antenna Module that will allow your DirecTV receiver to also get over-the-air channels. It plugs in to the USB port on the DirecTV reciever, but that is the only DirecTV device that will work with the USB port on the receiver. As of December 2013, the AM21 is not available on DirecTV's website, but third-party sellers still have them available for purchase (see link in Resources).
If, for whatever reason, your DirecTV receiver is unable to download updates to its software via the satellite connection, the USB port can be used by DirecTV technicians to load software updates to the receiver. Users aren't able to access this functionality and shouldn't try, since it will void the warranty and could result in the receiver being kicked off of the DirecTV network.
The USB port provides power for USB devices like mobile phones. You can connect your charging cable to the USB port and keep your phone topped up with juice while you watch television. Some users have even gotten a USB keyboard to work so that typing things into the search tool is faster. Overall it's not a huge boon, but it does provide some functionality for the lonely, forlorn USB port.
The GenieGO DirecTV receiver allows you to download shows to your mobile devices to watch on the go. It also has a USB port that will support an external hard drive to add additional storage capacity. If you plug a USB hard drive into the USB port on the DirecTV, GenieGO will reformat the drive -- deleting everything on it -- and format it for use only with the DirecTV system. If you want to use the drive for anything else you will need to format it again, which means you'll lose those stored shows.