You can always add text objects to a scene (using the Add menu) but they are static and cannot be changed during the game. The score obviously changes during a game and so you need dynamic text to display it.
Tutorials created recently have been experimenting with the game engine’s input and output facilities for making interactive applications (including games).
The updated version was created because of a comment from dberube4 about setting the transparency to alpha (making the background transparent) which means you do not have to set up materials and so a much shorter tutorial. You can still parent the display to a black background plane if you want to but that is straight forward.
This is the finished file from the tutorial textDisplay9a.blend
You can append the text display object from the file into a game you are developing that needs a display. I show how to do this is the final part of a tutorial on developing a practice level of simple ball game.