A USB device like a memory stick or MP3 player can be plugged in so photos, videos, or music can be fed to the TV for viewing and/or listening. I use mine to power a FireTVstick so it turns on and off with the TV.
Some TV sets (e.g. Sony) also use USB ports for IR blasters. Which allows the TV set to control satellite receivers and other set-top boxes.
The video of course would have to come through other ports, e.g. HDMI.
As you see from the replies there are many uses for USB ports on TV's.
From playing media to updating firmware.
It's best to refer to the users manual for your particular TV.