In the same way that organisms need to clearly differentiate between cells that are part of them and not part of them, institutions need to differentiate between individuals who are inside and outside of the institution. Institutions are the second level of a group selection evolutionary system.
Therefore institutions need a way of differentiating an in-group and an out-group. Differentiating an in-group and an out-group is just gatekeeping. And since Organizations don’t do anything, people in organizations do things Gatekeepers are essential to Institutions. The need to differentiate in-group and out-group also means that all institutions are to some extent, ‘exclusive.’
Obviously “it’s complicated” as it is with biology! You gut is full of bacteria, etc.