The quick answer is when its needed.
A necklift is performed to tighten the neck contours and the underlying platysma muscle and a facelift lifts the jowls which sharpens the jawline.
In my practice, I perform a neck lift with a facelift about 80% of the time. The reason for that is that the neck results are wonderful and the combined procedure produces fantastic long term results.
I have had the opportunity to wrok on alot of secondary facelifts, so these are patients who had facelifts about 8-10 years ago and come in for a touchup. Their biggest complaint is that their neck has fallen. They usually don't specifically remember if they had a neck lift but on examination, the neck was not done at the initial surgery. So I am usually performing a revision face and neck lift.
For those patients who had a face and neck lift done- they usually don't need a revision neck lift and I end up just revising the facelift. Usually a smaller overall procedure than the first time.