1. To have regular non pon farr sex with, if he could get over his anxieties.
2. For pon farr so he doesn't die.
3. TO SOCIALIZE HIM. He's a dork, he needs training.
4. To bear him children, for the good of the ship and to give him status over Harry Kim who won't be breeding during this scenario.
5. To cajole him into getting rid of that Vulcan dork hair style.
After 20 years he will be on the holodeck in a Hawaiian shirt if he has the right woman.
So really it's only one reason, TO IMPROVE HIM.