Before we built swaps we asked forty cafe teams to keep a note of every change to their rota for a month. Most changes were agreed between two staff in a group chat and passed to the manager late in the evening.
The manager rarely said no. What they wanted was to know, and to be sure the person taking the shift could do it. That is why swaps ask only trained colleagues and keep approval to a single tap.
We also learned what not to build: automatic swaps with no manager at all. Not one team asked for it.