I'm sure it's different at different schools. However, I have heard that some adcoms care more about secondaries for a few reasons:
1. They are what the school actually wants to know
2. They aren't as polished as the PS. People spend months on a PS, making it perfect. If you're only allowed 2 weeks or something for a prompt or two, it's not gonna be as good, but it will give them a better sense of who you are.