

That’s how most EU regulations are created. They take the best parts of the legislation of various members and combine them.
As for weapons, harmonisation is a thing. However, the exact use cases will vary for different countries. A tank that’s optimal for Spain isn’t necessarily the best for Germany. Neither country wants suboptimal equipment. What is easier to harmonise is ammo, a fact that NATO have been exploiting for a long while.
There are also the implications. Before now, military has been done on a per country basis. If they want to move as a block, they need individual countries to step up. It also allows countries to act independently if desired. A unified army is seen as a threat to the sovereignty of individual countries.
Taxes. The EU introduced rules of tax transparency. The UK left the EU the day before the rules kicked in.