Under what circumstances can the hotel upgrade the room type for free?

Room upgrades are generally foreign hotels or domestic high-end hotels. Free upgrades generally have the following situations:

1. When you arrive at the hotel late and only stay for one night, and the hotel has superior rooms available, in this case, the room is empty anyway, so it is better to bend the rules and let the guests upgrade their rooms for free, so as to surprise them and help the hotel realize word-of-mouth publicity. Why not?

2. When the reserved room type is in short supply and there are spare rooms at the higher level, the hotel will upgrade for free in order to retain guests.

3. In case of hardware damage, ants or other accidents, the hotel will upgrade for free in order to compensate the guests.