This will avoid inadvertent coin expenditures due to mis-clicks.
Especially important when buttons for coining are adjacent to non-coin start buttons.
Sometimes players play after they're tired and more prone to mistakes. The ability to avoid this mistake will cause players to feel more comfortable playing longer.
It will avoid anger and stress in players who coin something they didn't intend to. It will have no adverse affect on players who intended to coin. A "Don't show this again" click box could be included if a player wants to disable that function.