On the one hand, there are compelling reasons to question the legitimacy of our institutions: for example, our government is so captured by profit interests that it refuses to counteract the depletion and pollution of our common natural resources wrought by those interests at the expense of everyone else. That is not government, but exploitation.
On the other hand, without a belief in the legitimacy of our institutions, there is little reason for people to pursue anything but their short-term self-interest against other people. Without hope of non-exploitative government, the only rational choice is to be the exploiter, not the exploited, or at least to avoid complicity by disengaging and praying for good luck.
Therefore, we need some basis on which to distinguish when an institution serves the common good and when it serves concentrated wealth. We need some reason to work toward the former, not the latter. This must be the role of critique.