Because ZK doesn't have an exponential economy, each unit is important, even the cheap ones. There's also metal reclaim in battles, which tends to be about half

Because ZK doesn't have an exponential economy, each unit is important, even the cheap ones. There's also metal reclaim in battles, which tends to be about half of your econ. So fights become terf wars to make sure the enemy doesn't reclaim each battles' spoils.