They can define what is real by timing, exaggeration or understatement or filtering information. Under Nixon in the Whitehouse, Watergate events showed Erlichmann and Haldemann tightly controlling access to the President. Someone must manage at the boundary. Without it the organisation can atrophy becoming closed, isolated and ignorant. By managing boundary transactions the organisation brings in what it needs to be autonomous, defend itself and grow.
Recruitment is a good example.
Key skills are brought in and membership controlled. Those involved at the recruitment boundary can enjoy high status with those who join the organisation. Autonomous groups and departments who can defend their position are powerful. At the boundaries individuals compete for control. Those acting from self-interest, knowing or not knowing the effect they have on others, may threaten organisational autonomy and cohesion.