When a project is more complex than a single class, and it usually is, then it is wise to define a project structure. We will have to decide where we store the source files, where the resource files (the ones that contain some resource for the program but are not Java source) are, where the .class files should be written by the compiler, and so on. Generally, the structure is mainly the directory setup and the configuration of the tools that perform the build.
The compilation of complex programs cannot be feasibly done using the command line issuing javac commands. If we have 100 Java source files, the compilation will require that many javac commands to be issued. It can be shortened ...
