How are drushes economical?

Loomed villagers are strong enough that with slightly higher numbers they can defeat militia. They can also quickwall themselves into their resource collection.

Three militia cost 180 food and 60 gold. People say that fighting the militia with villagers costs a lot of idle time, but since your opponent loses that many resources, and earlier than you, it doesn't seem like a worthwhile trade most of the time. The villager seconds spent on training the militia can be worth more than the time spent fighting them.

Is it just to disorient your opponent, and make it harder for them to build their archery range/stable/etc? If they catch a lone villager and kill them it makes sense to me, and maybe there are situational benefits like intercepting a tower rush, but other than that I'm not convinced. Drushes are used regularly in high level play though so there must be good reasons.