The Arvus Lighter is a small, general purpose cargo shuttle that is widely used by the Imperium of Man for many tasks, both in space and on the surface of planets. The Arvus is commonly used by both Imperial Navy warships and Imperial merchant vessels, including Rogue Traders. The Arvus is just one of many types of small, void-capable shuttles used by the Imperium. The Arvus is piloted by a single pilot, and is not equipped with any weapon systems, in fact, the Arvus possesses only the most basic navigation equipment, external sensors, and proximity alarms. The Arvus has a powerful engine that allows it to carry heavy cargo to and from a planet's surface.
Of course that does not mean that the Arvus is limited to being a sitting duck, certain Imperial crewman are known to equip their Arvus with an improvised forward-firing twin-linked Multi-Lasers, twin-linked Autocannons, 2 twin-linked Heavy Stubbers and even two Hellstrike Missiles for self defence.
For some reason, the Lost and the Damned has a whole bunch of those. Other then having a quick form of transporting goods and resources, we can't really see that much use of a Arvus in a battlefield. At most the Lost and the Damned can used them as a reserve fleet to back up other allied flyers. Unfortunately, even then their piss poor stats makes the more like target practice to the enemy more then anything else.
TL;DR: It is a Trojan with wings.