Wstein is indeed right on it: by burning the board, all energies are released. Contrary to what well-intended posters suggest, energies DO attach themselves to ALL objects. Fire is cleansing, separating all elements and energies to a neutral state. For an Ouija board, in particular, there are concentrated energies. Whether or not they are harmful to you isn't really the point: as steward of energy, it is a kindness to let all of those energies free.
As well, when you buy things from a thrift store, try to leave them outside in the sun for at least a day, three days is better. This allows the energies to adjust, but also your home energies to 'check it out' and make adjustments, too.
When you dismiss things lightly with "Oh that's just..." you are neglecting important aspects of life and energy.