The trap bar is a terrific aid for the deadlift and is considered by many much superior to a straight bar. It reduces spine stress relative to the straight bar deadlift, and puts the arms and hands into a near-perfect position. It also makes the lowering of the bar a reverse of the ascent. With a straight bar, lowering the weight can be a problem. This is because the knees get in the way more on the descent than they do on the ascent. The extent of this depends on the individual structure. This rhombus-like bar lets users get the best from the deadlift while keeping technique problems to a minimum. It is an outstanding bar. All who use it will benefit.
If you are a competitive powerlifter and thus must use a straight bar at meets and in most of your training, a trap bar can still be a valuable "off" season training tool. It greatly reduces unnecessary stress but while developing strength that may have carry-over value to the straight bar deadlift (or even squat).