Why does Finland maintain a large military even though it has no chance of defeating Russia?

My answer on Quora.

For the same reason that you lock your doors when you go to work even though a well motivated thief can find a host of ways of breaking into your home if they really want to. It is a deterrent.

Also in the event that Russia does attack Finland - and we know that the Soviets had no qualms about doing that in the Winter War (1939) - a large and efficient military could inflict a severe casualty count on the Russians greatly reducing the political utility of such an attack for Moscow.