From Halopedia, the Halo wiki
The Covenant, although their ships are fully capable of reducing most UNSC vessels to heaps of molten slag in a matter of minutes, have also shown a desire to board and hopefully capture a human vessel, in hopes of capturing valuable information from their computers. Thus far, they have done so by using boarding craft. This has only been done rarely, such as at 23 Librae. Infantry are deployed into the heart of a vessel for purposes the UNSC can only speculate. Suggestions and evidence collated so far for motivations have been:
The boarding craft is a reusable vessel and is able to leave the UNSC ship and return to a nearby cruiser should the need to retreat occur. When the craft boards via windows it seems to use the outer edge of the boarding tube to superheat the glass, thus the glass does not shatter.
Types of boarding craft
Boarding craft were also responsible for the destruction of the ODPs Athens and Malta stations. Unlike in Halo: Combat Evolved, these boarding craft could be clearly seen through the station's windows. The craft used a long boarding tube to penetrate the windows of the station. The entrance/exit to the boarding tube was covered with an impenetrable energy shield that only allowed their considerable complement of Covenant soldiers out. The boarding tube seemed to function on a similar basis to the gravity conveyors found in the Covenant holy city High Charity. The tube, once deployed, had a superheated rim to melt glass instead of shattering it, therefore cooling down so air cannot escape.
Halo: Fleet Battles
Covenant Type-28 Boarding Craft
In Halo 2, players cannot enter the boarding craft even when the shields for the entrance are turned off.
List of appearances