what is the meaning of " mkdir linux-build"?

[hhw]

  I found these commands:
  $> tar -xvzf libosip-0.X.X.tar.gz
  $> mkdir linux-build
  $> cd linux-build
  $> ../libosip-0.X.X/configure
  $> make
  $> make install


Can you tell me why he "mkdir linux-build" and "cd linux-build"?
Why not he directly "cd libosip-0.X.X" and "./configure".