If a consistent export is started then the export session has to read data as it was at the beginning of the export, regardless of any changes made. Therefore any changes made to the tables since the start of the export must be preserved in rollback segments until the export finishes.

An inconsistent export does a similar thing, but only for one table at a time -- ie. Changes made to table B while table A is being exported need not be preserved in rollback. Table B's changes only need to be preserved once table B is being exported.