The series finale, Hell Bent, ended things with Clara as a waitress, the Doctor playing his guitar and recalling the story of his adventures with her, and all of it took place in an American roadside diner, which looked a little too familiar. There was a reason for that, and it's mentioned by the Doctor himself. He recalls being there before, thinking it was with Clara, before realising that no, actually, that was with Amy and Rory. Yes, this diner - which is actually a TARDIS this time around - is the same one used in The Impossible Astronaut, the first episode of Series 6, which sees Matt Smith's Doctor gather his companions there to watch him killed by a figure in a spacesuit. You could go into similar themes here, endings and beginnings etc, but really it's probably because they still had the set available, and saved themselves a bit of money.