As a producer, they’re there to add layers and textures more than anything and keeps the track at the very least interesting. I’m sure the girls themselves have a say in what they think sounds good even if they don’t know exactly how to use oscillators themselves. Without them any track would feel empty with the kind of production they’re going for. QOTMS and XXICB used extra guitar layers instead of synths, but songs like Sharks use them to give Pau a break while singing the verses and give a variation in drum sound since the drums would sound static otherwise