One reason to photograph at night in a tourist destination is, as I explained in Photographing the Bridge of Sighs at Night, to avoid selfie-stick-toting tour groups. Another reason is to present a different emotional aspect of the place, as in this image of the Piazza San Marco.
During daylight hours, and well into the evening in warm months, San Marco is of course jam-packed. Competing classical schmaltz bands strive to drive tourists into over-priced outdoor cafes. Public events are staged in the square. But at night, when it is foggy and chill, the piazza empties. Symmetrical lighting adds to the symmetry of the architecture, and it is possible to capture an entirely new view of the Piazza San Marco.