In other words, why fly from SEA to HNL via DEN, do you ask?
One three-year-old granddaughter's repeated cross-examination on the phone the other day, "Why were you riding those elephants," is harder to answer.
Let's put it this way:
SEA-HNL 2670 Miles (UA doesn't fly that route in any case)
SEA-SFO-HNL 3068 Miles
SEA-LAX-HNL 3504 Miles
SEA-DEN-HNL 4380 Miles
Any more questions?
Flying all the way in F while sipping the Mai Tais UA features on that route adds a lovely fringe benefit.
In fact, it's habit forming.