Activating a standby database (failing over) is a non-trivial operation with significant ramifications which should not be undertaken lightly. What happens during the activation is that an internal resetlogs is performed on the standby database, which alters its "internal configuration" and breaks its link back with the original primary database. it cannot then be put back into the partnership with the original primary database, which was effectively its parent, unless you can roll back the activation activity which can only be done with flashback database options which come as part of Oracle's Enterprise Edition. As most people using Dbvisit Standby are working with Standard Edition then this is not a possibility so post-activation they need to create a new standby database, connected to the one you have activated which is now considered the primary, and then perform a role reversal to transition the databases back to their original position in the configuration (primary running on primary server and standby running on DR server).
For certain operations a Graceful Switchover (role reversal) might be more appropriate than activation, and this does not require a rebuild or anything of the sort. You can read more about this here.