First of all, a defensive military CAN be used to increase your country's level. I assume that you are "Level 1" in the game. Click on that "level 1" and it will bring up a popup window with the requirements to advance to the next level.
One of the requirements is a defensive military, so having defense for the secured country isn't COMPLETELY pointless. You earn gold coins by increasing your level.
The gold coins that you earn can be used to buy game cash, to buy population, or to buy boosters for your country (though I believe that boosters should only be used in lieu of the standard 30 GC/month for extending your countries--something is not working right if you need more than 30 boosters in a game month).
Once you have earned all of the levels that you intend to earn, then you no longer need a defensive military for your secured nation. If you do NOT intend to conquer a country, you can disband your units and your garrisons, and sell your military equipment for game cash. If you intend to conquer C3s and build an empire, you can transfer defensive weapons to defend the new countries that are NOT secured. However, you will need to purchase OFFENSIVE weapons to do the conquering.