It did not find ANY channels? It should have at least found the analog channels, unless Charter has gone all digital in your area, in which case you would have already had a box.
Second, a digital tuner can mean two things: OTA or QAM. QAM is the hd channels your provider sends "in the clear", usually just the local broadcast channels. For anything else, like ESPN, HBO, etc, you would need an HD cable box (and pay for the HD package of course). For OTA, you would need an antenna.