Why don't you need a white wire when wiring a 240v a c unit?

White wire is usually used as a neutral conductor and a 240v ac unit has no neutral. Each leg of the 240 volts is a "hot" leg

However, very often a 12/2 or 14/2 NM cable is used to power small window ac units using 240v. In this case one of the conductors is white, but is not a neutral. It is good practice to mark the white wire with black tape so it is readily identified as a hot wire.