1) To make Purab look good, they ruined Armaan
2) To start changing that equation since the ending is a given, they made Purab look like a fool with that ridiculous exit.
3) To make Armaan look good, now they have to show him proving his goodness by helping Jassi resolve her problems. Billu serves his purpose by being the cause of that problem. Coz as hot-tempered as he is, he loves his daughter a lot, and all Jassi had to tell him was her concern for Bebe's health (which Jassi could have easily done by herself, she did not need anyone's help) and he would have realized why she did what she did. They have shown Billu's temper tantrums in the past too, but where Jassi is concerned, he always ends up listening to her side of the story coz deep down he has a lot of faith in and love for her. That billu would definitely have been sensitive to how hurt his "Queen Victoria" must be.
So even though they focused so much on Billu's extreme reaction (and in the process made him look bad), but it really wasn't about him. It was about the new improved Armaan.
Move over St Purab, it's now Arman devta's turn.