Rabeinu Yonah is answering a question mentioned by the Rosh (or by the Rabeinu Nisim, whom the Rosh quotes) earlier, concerning the Minhag to recite Kidush in the synagogue on Friday night. The question was how can we recite Kidush in the synagogue, if there is a requirement to make Kidush in the place in which one eats a meal? The Gemara answered that it is done for the guests who will indeed be eating in the synagogue. But what about nowadays, when there are no guests who eat in the synagogue?

Rabeinu Nisim, as quoted by the Rosh, says that indeed, such a Kidush recited in the synagogue is a Berachah le'Vatalah.

Rabeinu Yonah, though, argues and says that it is not a Berachah le'Vatalah. His logic is as follows. The requirement to recite Kidush in the place of the meal is mid'Rabanan; the Rabanan required that a meal be eaten in the place where Kidush is said. Mid'Oraisa, however, one is only required to verbally declare the sanctity of Shabbos. Therefore, the Kidush in the synagogue is said for those people who do not know how to make Kidush themselves, and we want them to at least fulfill the Mitzvah d'Oraisa, if not the d'Rabanan, of reciting Kidush.

The Rosh continues and says that the phrase, "Ein Kidush Ela b'Makom Se'udah," implies that one has not fulfilled the obligation to recite Kidush at all if there is no meal, and therefore he argues with Rabeinu Yonah. Furthermore, he points out that when the Gemara asks according to the opinon of Shmuel (who requires Kidush b'Makom Se'udah) why Kidush is recited in the synagogue, he could have answered simply to enable those who do not know how to recite Kidush to fulfill their obligation mid'Oraisa.

The Rosh, in the end of this paragraph, just adds that according to both Rav and Shmuel, one who hears Kidush in the synagogue has not fulfilled his obligation to recite a blessing over wine when making Kidush, and when he comes home he must recite a new blessing on the wine, as the Beraisa states that Shinuy Makom necessitates a new blessing.

Yisrael