The assisted pullup is often utilized for clients or athletes that do not have the strength to perform enough body weight pullups to include in their programming. I like to perform this exercise with a slight twist then what you may have commonly seen before in the weight room.
Stretching the band across the squat rack as opposed to hanging it from the pull up bar allows the athlete or client to maintain a more vertical posture. Thus removing the potential learning curve/awkwardness of the legs swinging out in front of the body.
Another variation I like for beginners is the Assisted Pullup Isometric Hold. This gives assist during the concentric and eccentric portions of the lifts. However at the top, the assist is minimal and the athlete can really focus on recruitment and tension under the lats. The other added benefit is increasing time under tension with this exercise, as with your typical rep schemes a beginner who lacks general strength will tend to fatigue to quick. It also is a great accessory exercise for advance lifters because it keeps time under tension high while decreasing total reps, thus eliminating some of the repetitive joint strain.