Well, we can ascribe Anakin being born there to chance and/or fate, because The Force.
Everything else that happened on Tatooine is more or less a direct consequence of that. Anakin returns to see his mother. Obi Wan and Luke get sent there so Luke could get raised by Owen Lars, Luke's grandmother's stepson. Leia goes there to reach Obi Wan. Darth Vadar goes there to find Leia. And so on. Only Han being there at the right time really needs additional explanation, and since he's a smuggler he goes everywhere, especially shitty, desolate backwater planets.