The information stored in the takeoff data is used by the FADEC to compute the N1TARGET for takeoff. As s10an wrote, the temperature entered usually corresponds to the ATIS temperature. After the engines are started, that temperature must be within 10°C of the T2SYN temperature, which is the synthesized TAT at the fan inlet. If it's not within 10°, the takeoff data is not accepted and an EICAS message will appear so stating. Think of the temperature entry as a manual crosscheck for the FADEC.