Bluetooth is a secondary way by which tablets communicate without wires. The primary way is the essential WiFi and through ports and a few tablets also have Near Field Communication (NFC). Bluetooth can enable tablets to wirelessly communicate with other mobile devices nearby, as well as to such as Bluetooth-enabled speakers.
The nabi DreamTab does come with Bluetooth 4.0 (as well as NFC and WiFi), enabling it to wirelessly communicate as mentioned. Although not essential, Bluetooth can be a useful extra that can expand the options available for a user, although it also tends to drain the battery faster, so it should be turned off when it is not being used.