The latter involves a lifetime, or more likely, multiple lifetimes of practice during which one's understanding of dependent origination is continuously deepened. In the course of this practice, it is quite likely that one makes discoveries about the mind, about existence, life, death, and rebirth, verifying one or another aspect of the dhamma. Although strictly speaking such discoveries aren't necessary, they are just likely to occur. What is more, although "seeing" rebirth is strictly not necessary, rebirth itself is necessary for you or anyone to be able to walk the path to its end. If you are not reborn, then when you die, that's it. Game over. Mission failed.