What is the significance of setting subnet mask and default gateway while going online with a network?

subnet mask is important to difine the network part of the ip address and using it will let me know if I will communicate with a device in the same network or on another network...if i'm communicating with a device in the same network then i don't need a default gate way but if i want to communicate with a device in another network i'll send the data to the default gatway to determine the next hobe using dynamic or static routing.