>Does the ambassador have to stand in an office while getting lectured by a minister? What’s the purpose of this?

No. The ambassador has no obligation to go anywhere. When a country summons an ambassador, it's a request, not an order, and the means is the message. In other words, the act of summoning the ambassador itself conveys that the host country is unhappy with something that the country represented by the ambassador has done. It's diplomatic theater meant to sent a message to the ambassador's country. Whether the ambassador actually attends any meetings is up to them, although not going is frowned upon and could lead to further diplomatic steps.