If you configure an alternate path through the network in case the active path fails, you may not want traffic to revert back to the failed path, even if it is no longer failing. When you configure a primary path, the traffic switches over to the secondary path during a failure, and reverts back to the primary path when it returns.
At times, switching traffic back to a primary path that has previously failed may not be a particularly sound idea. In this case, only configure secondary paths, resulting in the next configured secondary path establishing when the first secondary path fails. Later, if the first secondary path becomes operational, the JUNOS software will not revert to it, but will continue using the second secondary path.