Software Assurance is an optional product Microsoft offers that covers your license purchase for a period of two years; during the time the software assurance is in effect, you would be entitled to any upgrades to the covered product at no additional cost, thereby guaranteeing you are always working with the most current version of the software. After the two year period, you may choose to renew the software assurance in order to continue to receive coverage.
The reason we strongly recommend software assurance for the Windows Server operating system, as it requires more consistent upgrades to keep up with the changing security landscape. Other products like Exchange or SQL frequently do not need these same updates as frequently.