Disc brakes are not a "need" item. Under the right circumstances (long steep descent with lots of switchbacks), I'm sure that an inexperienced rider could overheat brakes - rim or disc. If you overheat a rim brake, you risk blowing a tire off. If you overheat a disc, it will fade. Under worst case conditions you might need both a rim and a hub brake on the rear wheel.
Disc -vs- rim is more of a preference than a need.