Why do we need to include both the .h and .cpp files , while we can make it works with making just a .cpp file and then including it .

For example, Creating a file.h containing declarations, then creating a file.cpp containing definitions and including both on the main.cpp.

Or, creating a file.cpp containing declaration/definitions ( no prototypes ) Including it on the main.cpp.

Both worked for me , but still can't the difference since i do not have a background on the compiling and linking process .