I have a friend in our local police department, and everyone there had to be tazed as part of their training. I think it has to do partially with giving an understanding of how the tazer affects the one being tazed - so it's not used lightly. Plus, I think it's also due to having first hand experience in case they are accidentally tazed in the field (like "friendly fire"). Having experienced it once, they'll be better prepared in the unlikely case that would happen.
All I know from my friend was that it hurt like hell. He didn't really elaborate much beyond that. But everyone in his group was fine.