In the underutilized capital scenario, idle capital incurs costs, but no benefits. It must be destroyed. The easiest way to maintain the facade is to send the capital to war. If you acquire new land, congratulations, the "investment" paid off. If it doesn't, then the destruction of capital at least maintains the profitability of domestic capital.
War is really that simple. If you had a war for any other reason, everyone involved would see the stupidity after the first few skirmishes.