Why turtlesim needs to check ROSBUILD should include rosbuild.cmake or not? [closed]

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What's the meaning of first 4 lines in the following CMakeLists.txt ?

 if(ROSBUILD)
   include(rosbuild.cmake)
   return()
 endif()
 cmake_minimum_required(VERSION 2.4.6)
 include($ENV{ROS_ROOT}/core/rosbuild/rosbuild.cmake)

 # Set the build type.  Options are:
 #  Coverage       : w/ debug symbols, w/o optimization, w/      code-coverage
 #  Debug          : w/ debug symbols, w/o optimization
 #  Release        : w/o debug symbols, w/ optimization
 #  RelWithDebInfo : w/ debug symbols, w/ optimization
 #  MinSizeRel     : w/o debug symbols, w/ optimization, stripped binaries
 #set(ROS_BUILD_TYPE RelWithDebInfo)

 rosbuild_init()

 #set the default path for built executables to the "bin" directory
 set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin)
 #set the default path for built libraries to the "lib" directory
 set(LIBRARY_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/lib)

 rosbuild_genmsg()
 rosbuild_gensrv()

 find_package(wxWidgets REQUIRED)
 include(${wxWidgets_USE_FILE})
 include_directories( ${wxWidgets_INCLUDE_DIRS} )

 rosbuild_add_boost_directories()
 rosbuild_add_executable(turtlesim_node src/turtlesim.cpp src/turtle.cpp      src/turtle_frame.cpp)
 rosbuild_link_boost(turtlesim_node thread)
 target_link_libraries(turtlesim_node ${wxWidgets_LIBRARIES})

 rosbuild_add_executable(turtle_teleop_key tutorials/teleop_turtle_key.cpp)
 rosbuild_add_executable(draw_square tutorials/draw_square.cpp)
 rosbuild_add_executable(mimic tutorials/mimic.cpp)

Why turtlesim needs to check it and include rosbuild.cmake?

Thank you~