Owen and Beru Lars are Luke's uncle and aunt. Owen is a moisture farmer on Tatooine. We're not totally up on the details of moisture farming, but he lives on a desert planet, and people and creatures need water to survive.
Owen and Beru's home represents a place of safety for Luke. Before we come to the Lars' farm, every other setting has been dangerous in some fashion. The spaceships were locked in battle, the desert was harsh and unforgiving, and the Jawa Sandcrawler was a horror house—the droid equivalent of one anyway.
The Lars' farm provides the first setting where the viewer can breathe. We see families eating dinner (and drinking blue milk), old men complaining about their work, and young men whining about power converters. In the Star Wars universe, it's as close to our ordinary world as we get and it provides a place of respite from the insanity of the harsh world we've previously been witnessing.
These people are doomed.
Searching for Jungian symbols in Star Wars, James F. Iaccino notes,
"Luke's uncle resembles the father archetype in that he needs to keep things the way they are, with himself in charge. The individuality which Luke is showing cannot be tolerated because it threatens the entire structure Owen has imposed over his family."
This read of the character makes sense. A no-nonsense surrogate father, Owen is stern with Luke in all of their scenes together. Consider this telling exchange at the Lars family dinner table:
LUKE: But what if this Obi-Wan comes looking for him?
OWEN: He won't. I don't think he exists anymore. He died about the same time as your father.
LUKE: He knew my father?
OWEN: I told you to forget it. Your only concern is to prepare those new droids for tomorrow. In the morning, I want them up there on the south ridge working on those condensers.
They're talking about Luke's deceased father (whom he's never known), and Uncle Owen's response is, "Forget about it and get back to work."
Owen is clearly trying to keep Luke's family history hidden, perhaps knowing that Luke's discovering the truth will jeopardize any chance Owen has of keeping him under fatherly command. Of course, Owen's unwillingness to deal with the world beyond moisture farming leads him to be unprepared when the universe comes a'knocking in the form of blaster wielding stormtroopers.
However, we do have a question about Iaccino's reading of Owen: is Luke's individuality threatening to Owen because it might subvert the family structure—supplanting Owen as head honcho—or because it might ultimately harm Luke? Put another way, is Owen trying to harness Luke's individuality for himself or for Luke's wellbeing?
BERU: Owen, he can't stay here forever. Most of his friends have gone. It means so much to him.
LARS: I'll make it up to him next year. I promise.
BERU: Luke's just not a farmer, Owen. He has too much of his father in him.
OWEN: That's what I'm afraid of.
If Owen is trying to reign in Luke's individuality for the boy's own sake, then he's already lost that fight, and Beru knows it.
Bringing Down the House
As required by the classic hero story, Luke's home must be either be destroyed or threatened as a catalyst for the hero to accept the quest. You can see examples of this wide-ranging trope in the Shire of The Lord of the Rings and in the warren in Watership Down . Even Superman's origin story riffs on this trope, only instead of a little super village it's the entire planet of Krypton that gets hit with the proverbial hammer.
After learning that the Empire has picked up the droid's spoor, Luke realizes that will lead them to the farm, but he arrives too late. The farm is a smoldering ruin and the charred remains of his uncle and aunt have been unceremoniously left to rot.
Before Luke had decided not to join Obi-Wan, but the sight of the Empire's handiwork changes something in him. He returns to Obi-Wan and confesses:
"I want to come with you to Alderaan. There's nothing for me here now. I want to lean the ways of the Force and become a Jedi like my father."
True to the trope, Owen and Beru's death lifts the barrier for Luke to leave home and the hero to be finds his path to his destiny.