It's more efficient for them to donate to 100% things they want, instead of volunteered taxes which go into the big bucket. But, if everyone volunteered more ta

It's more efficient for them to donate to 100% things they want, instead of volunteered taxes which go into the big bucket. But, if everyone volunteered more taxes, the donations are now more efficient, because they represent compromise on what to spend taxes on. (Alice gets stuff Alice likes, Bob gets stuff Bob likes) Forcing everyone to "donate" prevents others from defecting. Also, "If you don't like war, what's stopping you from laying down your weapons?"