Alright let me explain this
The risk is that if mafia end up in the spared group, they get to kill two people that night.
Of course, there are people who are arguing that we can successfully spare 4 town, and the risk for us not being able to do that is we get put into a state where we have to name the mafia with no information or instantly lose.
Anyway, this happens night 4, and mafia will have killed, presumably, 3 people during the night. If we kill 2 people as well, then there are 6 players remaining alive. If we have killed no mafia, and mafia is spared, then we lose because that instantly puts us to 4 alive.
If we instead spare 3 then that leaves 7 players alive, and we get put into a situation where we know how many mafia are in 1 group of 3, and by extension how many are in the other group of 2-3