Landas are basically the Orks' equivalent of the Space Marine's Thunderhawk gunship in both role and utility. As a Landa is a name for any large transport aircraft big enough for Orks to use, there will be some slight discrepancy and variation on what exactly counts as a Landa, since like most Ork vehicles, there is very little standardization of it.
The main role of a Landa is as its name implies, to land troops and smaller vehicles from orbiting spacecraft to the surface and to quickly move them from one battlefield to another. Landas are a classic example of Ork design, being huge ungainly craft brutally efficient in their task. Ork Landas are slower and more ponderous than any Ork or Imperial Aircraft, but it would be a fatal mistake to consider one easy prey. While not nimble, Ork Landas are extremely tough and can ignore damage deadly to most aircraft. Landas are equipped with numerous gun turrets and rokkits that allow them to lay down an impressing rain of fire against enemy aircraft that gets close enough or to support disembarking troops once on the ground.
Once the Landa hits the planetary surface, like the Thunderhawk, it would open up its 'jaw' to reveal its cargo to the front lines of battle. Usually, a good group of Ork Boyz and a couple of light vehicles like a Junka, Looted Wagon, Warbuggy and/or Wartrakk are the standard 'cargo' that a Landa transports. Although a Warboss and a Deff Dred are also a likely passenger too if the duty calls.
They are a somewhat rare component of an ork invasion force, however. Firstly, the warboss needs to be convinced that either they are going to take more than one planet or that there's a fight to be had in space and thus that they should leave their main fleet in orbit. Otherwise, the orks are likely to simply crash the main fleet into a planet. Secondly, the Landa competes with transport rokks and teley-portaz for deployment; the former being considerably more smashy while the latter is more kunnin'.