Not very, for a typical applicant coming from a large, affluent public school, barring something flagged in it.

Colleges know the counselors usually don’t know the kids well. It’s a place to provide additional information and context, which for some applicants might be very, very important, but for many it’s not.

Students generally fill out a questionnaire or write up a summary about themselves for the Counselor. At D.C. large public school, the parents did the same. The counselor used that and presumably the school record to write the recommendation.

How important are they? We're at a large public and our former school counselor retired. The new person does not know any of the seniors at all.