Imagine a small island nation in a stable region of the world with very little in the way of natural resources. External borders are not in dispute, the closest nations are a sea-ride away and all are 'friendly'.
Why would such a nation throw a lot of money at defence?
Perhaps it’s paranoid. A peaceable small island nation (with very few resources) is not an obvious target for invasion or attack.
Or then again, perhaps it isn’t peaceable. If a country makes itself a lot of enemies it might well also need to make sure it could ward them off. With guns and tanks and high-tech fighter planes. A Trident missile might be useful.
The UK throws a lot of money at ‘defence’. Nearly $1 billion of taxpayers’ money is thrown at NATO, for a start. That's more than the total GDP of Indonesia - another island nation, incidentally. (But it has 4 times our population and a lot of highly coveted resources).