Well historically it's been to thank someone for walking all the way up to the stage, overcoming their fear of focused public attention and just 'being a sport.'
Lately however I'm seeing a lot of magicians on youtube video in "casual interaction." This seems to involve:
- Magician approaches unsuspecting spectators (usually wearing black to aid in the ambush)
- Magician either asks if he can show them something (cause he's a flasher) or try an experiment (cause he's actually a lab technician)
- Magic shows them something (a selection of completely ordinary 1960s ESP cards for example)
- Spectator looks "confused" (muggle word meaning AMAZED)
- Magician nods and gushes 'THANK YOU' (safely putting to rest any question of wether the aforementioned experiment was a success)
Now it seems to mean, "thank you for helping me show you I'm special. You do believe me, don't you? BELIEVE. BELIEVE. MIIIND FREEEEEEAAAK."