Roles result from role hierarchies. The role hierarchy is a security tool that opens a vertical access to records that users do not own. Users at any role level can have a read only or read/write access on all records and related records owned by or shared with users below them in the role hierarchy.
The role hierarchy applies only if the default values are restrictive (private or public read only). If organization-wide defaults (OWD) are public read/write, it is not necessary to open additional access to records.
Profile object permissions overrides role hierarchies.
Example: A Marketing Manager has read-only or read/write access to all records of his subordinates (telemarketing managers, telemarketers), even if he does not own the records.