Summary
When a UNSC ship is under seige by the Covenant Fleet, rather than simply destroy it, Covenant forces may attempt to board the ship and search for vital information. In such a situation the Boarding Craft is used. They simply use the empty escape pod docking rings to board the vessel. Humans go out, the Covenant come in.
The Boarding Craft are reusable, and able to leave the UNSC ship and return to a nearby cruiser should the need to retreat occur.