Gul has intended this track to occur since the first episode. As she said in an interview - this show is her baby. Why? There's much more to QH than just a love story.
But I won't go into that until this Friday - when I have prepared a grandiose post like my 'Sacrfice, Sugar, and Catharsis' post.
To answer your question as briefly as possible:
a) Like Tanvi said, provide a veritable reason for the brother clash. Ayaan is falling for Zoya - he will harbor bitter feelings against Asad, and Asad will develop hatred for Ayaan due to his mistrust of MaBurt. Hugs don't make them not have clashes Tanvi 😆 - Tanwh**e also hugged Asad - did he hug her after the truth came out? No. :P Same with this. The brother relation will be clouded over Zoya.
b) Provide veritable punishment for Razia. Her daughter will never be her husband's first love. Ayaan will resort for Humaira after sacrificing Zoya for Asad. Razia has done many sins. Gul has made this track to answer for Razia's sins. And sadly, they will be played out through her daughter Humaira.
c) Ghafoor-Zoya. The Abu track will happen - and MaBurt introduced Ghafoor to Zoya. Now that AsYa are reuniting, let us see how Ghafoor will continue to investigate Zoya and figure out he's her father. That is the next obstacle for AsYa.
In essence, this track was necessary for the gist of the story.
It had loopholes in execution - but I admire Gul. This track was eagerly awaited and did bring out the powerful stuff for what's ahead.
Hope this answers your question :)