Say one wishes to transmit a sequence of characters - "ABC". What are the steps that are invariably done to enable this transmission of information?
This is too complicated for any engineer to handle. That's why we divide communications into layers. 

The bottom layer is invariably the physical layer, voltages on cables, or radio signals in the air.

The top layer is the application, 'let's send 'ABC' to Fred'. 

In between are half a dozen layers that handle collisions with other data, addressing, error correction, no loss of data, no duplication of data, making the data arrive in the right order, make sure the data goes to the right place, talk ASCII or unicode, make sure any bad guys can't see the data, etc.

Generally we actively engineer one layer, use the services of the layer below, and provide services to the layer above. Then we can get our head around what's going on.