If you were using an Ethernet switch or hub, the crossing over would happen within that device. However, if you don't want to bother with introducing that external device, with its power requirements, you can use a crossover cable. Another use for the cable would be to connect two hubs together.
Similarly one may ask, what are straight and crossover cable?
An Ethernet crossover cable is a crossover cable for Ethernet used to connect computing devices together directly. By contrast, patch cables or straight through cables are used to connect devices of different types, such as a computer to a network switch or Ethernet hub.
What is straight through cable and crossover cable?
Straight-through cable is a type of twisted pair copper wire cable for local area network (LAN) use for which the RJ-45 connectors at each end have the same pinout (i.e., arrangement of conductors). Crossover cable is used to connect two like devices without the use of an uplink port.