According to my old folks, they told me, this live, become a
monk or nun, is for next life. Next life, they will be free of
sufferring when they reborn.
Actually, the purpose is to be free from suffering in this very
life, and many practioners, monks included but lay-men included
also, achieved this goal in this very life.
Also if that person did not achieve enlightenment in this
lifetime, it doesn't mean he would achieve enlightenment in the
next. But the practice continues.
Also the Buddhist goal is not to reborn in a realm of heaven as
even that is temporary and subject to impermanence. The Buddhist
goal is to end the uncontrolled cycle of rebirth and suffering. So
we do not look to the next life to be free of suffering. Any future
births as more bondage.