On Tatooine outside Luke's house, there are these small tower-like objects outside Luke's home. There are 3-4 of them.
The same thing can also be seen outside of Obi-Wan's house as well.
What are these and their purpose?
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Those are moisture vaporators. These are quite literally the entire point of the moisture farm, to collect atmospheric water vapor. According to the Star Wars: Official Fact Files, a single vaporator can collect 1.5 litres of water per day.
The one outside Ben's house (an abandoned moisture farmer's hut) is a low output vaporator, one suitable for a single house.