Can be used to neutralize religion.
If you do not want your neighbor to receive bonuses from your religion at this stage, then go to him with these missionaries. Likewise for any city with any religion other than the one bestowed by these missionaries. You make a city non-religious and the city stops exerting pressure on a very decent territory.
You can sail to the cities of Defender of the faith or Crusader Spirit and instantly remove defensive or offensive bonuses.
You can convert your city, get a new religious building, and then restore your religion.
The peculiarity of buildings is that they reinforce the dominant religion, and not the one that allows them to be acquired. At the same time, your city will receive additional protection of 10% from the pressure of foreign religions and will be more difficult to convert in the future.
Or use them as traps so that the AI tries to capture them back, luring combat units out into the open.
But any use of foreign missionaries will speed up the adoption of the Reformation, as there will be more believers. But it can also slow down the Reformation of someone else, since again there are more believers.
It is possible that you will even receive bonuses as a distributor, but I am not sure.