The file makes it so that gangs hate each other, and cops dislike gangs. So when a gang ped sees a rival ped, they'll attack them. Combined with the modified popcycle that increases police and gang presence throughout the city (so you'll see more gang and police cars in traffic/on the streets), it leads to more emergent firefights.
We actually used to have a version of the relationships file that had cops also hate gangs rather than just dislike. Led to even more constant chaos, with neverending sirens as cops would actively hunt down gang members in the nearby vicinity.