Humanitarian organizations often operate with less budget. In order to make every penny count, they won't let you spend your time on useless tasks. You will most certainly make a difference. When you work on the operational side, such as fieldwork, you can see the impact that you and your team have on a day-to-day base. When you work on a more strategic level and spend most of your time doing research or working on long-term projects, your impact won't be as direct as the impact of your colleagues in the field, but you did contribute to that same impact. And remember, Rome was not built in a day!