In my view, the main contribution of Critical Security Studies is a focus on something other than states and formal actors such as the United Nations. I would argue that Critical Security Studies has encouraged us to think more seriously about things that affect individuals, particularly from unprivileged groups in the world.
For the most part, realism and idealism (the main perspectives on security studies and international relations in the past) have focused on elites and/or on formal institutions. Realism believes that the only actor that matters is the state. Idealism sees the state as important, but also looks to multinational organizations like the UN or to NGOs like the Gates Foundation. These are all institutions that are definitely part of the power structure in the world today. They are important, but they are not everything.
Critical Security Studies encourages us to look beyond these relatively elite groups. In Critical Security Studies, we are asked to think about things that affect groups that are decidedly not part of the elite. We are asked to think about how certain aspects of our world order affect people like farmers living Chiapas in Mexico or like the Uighurs or Tibetans now being oppressed in China. Critical Security Studies asks us to think not only about things that affect the security of the United States or any other country. It asks us to also consider things that affect the security of these oppressed groups. It asks us to think about human rights and how our systems affect those rights. In this way, Critical Security Studies makes an important contribution to security studies. It encourages us to think about the world as something more than a collection of states and elite groups. This is an important thing for us to keep in mind.