Today or in the near future, plant a bomb anywhere in public in a city and you're likely to end up on camera.
If a terrorist can accomplish the same outcome while not killing their self (suicide), they get to keep killing; it net lowers the cost of terrorism dramatically. The biggest cost in suicide bombings directed at the developed world, is getting the bomb carrying person into location (immigration, housing), feeding the person, and the time it takes to arrange it all. Drones mean you bring one person in, and they get to take action potentially numerous times.