It exists for reach. A mentor's hours are fixed, and a group format is the only way one of those hours serves eight people instead of one. It also adds something one-to-one cannot: members learn from each other's questions, and comparison across a cohort makes progress legible in a way an isolated engagement does not.
What it costs is administration. Seats, waitlists, drop-outs, shared materials, who attended which session and who is behind — all of it is bookkeeping that grows with the group, and none of it is mentoring.
Cohort formats are common in sponsored and employer-funded programmes, where the funder is buying reach per unit of budget. That makes per-member progress tracking a reporting requirement rather than a nice-to-have.
Mentor10
Mentor10 runs cohorts on the same rails as one-to-one work — seat management and waitlists, shared curriculum and agenda, group sessions, private notes, and per-member progress reporting.