Difference with "traceonly explain", "autotrace on" and "traceonly statistics"

Explain plan is supported by default:

SQL> set autotrace traceonly explain

But reporting statistics requires plustrace role.

SQL> set autotrace on
SP2-0618: Cannot find the Session Identifier.  Check PLUSTRACE role is enabled
SP2-0611: Error enabling STATISTICS report

SQL> set autotrace traceonly statistics
SP2-0618: Cannot find the Session Identifier.  Check PLUSTRACE role is enabled
SP2-0611: Error enabling STATISTICS report

SQL> get ?\sqlplus\admin\plustrce.sql
  1  --
  2  -- Copyright (c) Oracle Corporation 1995, 2002.  All Rights Reserved.
  3  --
  4  -- NAME
  5  --   plustrce.sql
  6  --
  7  -- DESCRIPTION
  8  --   Creates a role with access to Dynamic Performance Tables
  9  --   for the SQL*Plus SET AUTOTRACE ... STATISTICS command.
 10  --   After this script has been run, each user requiring access to
 11  --   the AUTOTRACE feature should be granted the PLUSTRACE role by
 12  --   the DBA.
 13  --
 14  -- USAGE
 15  --   sqlplus "sys/knl_test7 as sysdba" @plustrce
 16  --
 17  --   Catalog.sql must have been run before this file is run.
 18  --   This file must be run while connected to a DBA schema.
 19  set echo on
 20  drop role plustrace;
 21  create role plustrace;
 22  grant select on v_$sesstat to plustrace;
 23  grant select on v_$statname to plustrace;
 24  grant select on v_$mystat to plustrace;
 25  grant plustrace to dba with admin option;
 26* set echo off
 27