One of the primary reasons to reactivate a license on a BIG-IP system is in anticipation of upgrading the system to a new software version or to activate newly licensed features. To upgrade, the service check date in the license on the BIG-IP system must be later than (after) the release date of the software upgrade version, indicating you have an active service contract with F5 and are therefore eligible to do the upgrade. (You can find the official "release date" for a particular software version in the BIG-IP Release Matrix.)
To reactivate a license automatically from the BIG-IP system, the system must be connected to the internet. Otherwise, you must use manual reactivation. Manual reactivation involves generating the dossier on BIG-IP, getting the dossier up to the F5 License Server (copy/paste or download/upload), generating the new license, and getting the license back to the BIG-IP system for installation (copy/paste or download/upload).