Imagine that one user makes a query on a table during an updata by another user. To provide consistency, the first user will query data being updated from ROLLBACK SEGMENT, until the second one commits his transaction. Just an example.

REDO LOGS has recovery purpose: all changes made to the database (commited or not) are saved as redo records on online redo logs. If the instance fails, or anything else, one can recover transactions until the failure time.

If the database is always ok, redo logs will only be written until they get full (so they are cleaned and started to be written again). They work as a complement to backup, in other words.

Rollback segments are different, they are heavily used during updates/queries, not just being written but readed too, when it's needed to provide a consistent query.

Hope I was clear, not sure.

Adriano.