Although all UFOs used by the aliens of the First Alien War only needed a power source and navigation system for the bare bones(excluding the Small Scout for some reason), certain UFOs were fitted with special componets and machinery that is used for specific tasks. For example, a Harvester is fitted with surgical equipment for mutilating livestock and containers for storing harvested foodstuffs. Of course, X-Com could not use these in their own reverse-engineered UFOs, nor did they actually need to, but they were useful for earning extra cash by selling them on the Black Market.