It provides the mechanism for every parent to be in contact with an adult in the school before there is a problem. It's not homeroom, which is about organizational needs. Effective advisories promote consistent, long-term relationships. The school's design must allow for advisor-advisee relationships that last for several years. Advisories can also facilitate relationships between students by including members from several or all grades in a school.
Younger kids learn from older kids and older kids like to be informal mentors.
Students pass on wisdom about the school's culture and norms, like how hard it is to be a junior, that it's worth it to hang in there, and that it's dumb to skip class or get high before school. Ninth graders will benefit the most from mixed age advisories, so don't isolate them from the older students. Advisories work best when every certificated staff member is responsible for one. When people aren't involved, they get left out of important discussions. Having as many people as possible be advisors is also important in reducing the number of students in each advisory.