That's part of it, sycld, but another big thing is that it gives the mafia a chance to react to the information gained at the end of the day phase before the town can coordinate their actions. Early in the game where information is sparse, it doesn't make much of a difference. But when the town has more info and has killed a mafia or two in the late game, the mafia's only advantage is coordinating night activity before the town can.

The town can plan it's actions during the day phase, but the outcome of the lynch may make those plans unusable. They dont get a chance to change their plans together if they cant PM at night. The mafia's advantage in the late game is that it can adapt to the information despite its smaller numbers, and it can react to pre-planned contingencies.

For what it's worth, the town usually wins. Depending on the model, it's sometimes a severe favorite to win. I think as we play more games with the same people, we will start to see the town winning because the game favors honest players in the long term. Allowing PMs helps the town in general, but preventing them at night helps balance it for the mafia.