Having done a Practice Pan on 121.5 during my QXC leading up to PPL, I didn't realise I was drawing the wrath of those posting above. You see, it tells you to do exactly what I was doing in the standard text book. It also tells you, as "RTFM" confirms above, that D&D like to encourage students to give it a trial to experience it. Come on, guys, you must have been through this on your own PPLs once upon a time.
What the PPL books don't tell you is that every commercial aircraft is maintaining a watch on the frequency. You are given the impression it is only you and the D&D controller.
I think this reason for Practice Pan calls is much more likely than the type "BOEINGBOY1" suggests, of a GA pilot lost but not wanting to admit it!