here's clear explanation from bahasakita http://www.bahasakita.com/yang/

the correct one is mobil itu melaju sangat kencang/melaju kencang. it's uncommon to say mobil kencang. Ada juga mobil sport, mobil sedan,etc. We can say "yang" to explain noun, example: Mobil yang melaju sangat kencang itu milik Direktur perusahaan B. Anak kecil yang berkepang dua itu adalah tetanggaku.

"Yang" used to explain which car. Maybe its kind of like "Annemi seviyorum" The explanation of "yang" could be the same with the explanation why do you put annemi not just annem. Hope it helps

"yang" is like a complement. It makes a sentence be better.