When there is a need for a device on one network to communicate with another device on another network, of course with a known IP address, the point of exit is through default gateway. The default gateway can be likened to the gate of a particular house A. To go from this house to another house B, you need to go through the gate.
Default gateway permits outbound connections from one network to another for the purpose of communicating.
Note that it is called "default" because unless otherwise specified, that is the point through which outbound connections to another network is possible.
