Tzatziki is typically served as one of the many small appetizer options in a meze, which is a Greek multi-course meal that provides a sampling of a wide range of dishes --- and leaves diners typically stuffed to the point of discomfort.
And it’s also served as an option along with pita bread, as a kind of dip. And I personally like it with Greek souvlaki (pork shishkebob) or with souvla (lamb shishkebob).
But I know and love it best as a topping for falafel, the wonderfully flavorful and savory chick pea patties that are mainstay of Middle Eastern cuisine.