Boom Beach Support guide: Armory
If you want to do well at Boom Beach, you'll need to take over a load of islands.
If you want to take over islands, you'll need a strong army.
If you want a strong army, you'll need to level up your troops and Gunboat.
If you want to level up your troops and Gunboat, you'll need the Armory.
It's as simple as that really. The Armory allows you to make the troops you have access to even stronger than they already are, and improves the weapons and items that are sent to the battlefield via the Gunboat.
It costs Gold to upgrade these, and there are strict level limits in place, meaning that you won't always be able to improve your forces all of the time, but each improvement is a welcome boost to your combat strength.
Even if you're a passive player, it's worth building, since every improvement made also adds to your overall Experience.