Why add "QT + = Widgets" to the pro file in QT5

Write "Qt+=widgets" in pro file to introduce qtwidget this module,qmake when generating makefile, will set up include path and Lib path, set libs at link time.

QT + = Widgets consists of two parts:

1:lib, this is the path of the Lib and the file name of the Lib file.

2:include, the path of *.h is set in the pro file, and the. h file name is set in the code # # #

QT + = Widgets Seemingly can be written separately (roughly):

include+= A string of paths (because filenames appear in the code as include, so there is no need to have filenames)

lib+= a string of paths +-l filename +-l file name ....

If only include <QApplication> in the source code only introduces the declaration, but does not have LIB, so the link will be wrong.
This is due to the C + + header file and lib separation of this design pattern, the modern language is basically used package management, with import to solve.

Why add "QT + = Widgets" to the pro file in QT5