کاربرد On which در متن زیر

سلام تو جمله زیر which ک قبلشم on اومده بخاطر چیه و چه کاربردی داره میشه توضیح بدین؟

And when he grew old he divided the kingdom among his four sons, that they should be the pillars on which the peace of the land rested.