Explanation:

Public Groups are the mechanism in Salesforce designed to bundle unrelated users—across roles, profiles, or territories—into a single sharing unit. Once defined, you can reference the group in sharing rules, manual shares, or Apex managed sharing to grant record access.

→ Role Hierarchy is strictly vertical; you can’t use it to group peers or cross-functional teams who don’t report to each other.
→ Sharing Sets are only for high-volume community licenses and grant access based on profiles and record owner, not arbitrary clusters of users.

Thus, when you need to share records with a specific team or committee that doesn’t map to the org’s structural hierarchy, you create a Public Group containing exactly those users. A sharing rule can then grant read or read/write access to that group.