For the great unwashed, starting an IV without proper training can be the trail to disaster. You can make a bad situation a whole lot worse incorrectly pokeing around in their hand or arm with a needle. Infection, nerve damage, air embolism,......etc.
If your doing it on someone else, there is the potential for legal issues should there be anything but a completly positive outcome should you not be currently certified.

"There is nothing so frightening than ignorance in action."