In addition to Derricks good advise Spanish cedar is also naturally resistant to mold and mildew which is important in a humid environment. Resistant doesn't mean immune but in instances when mold does occur you will see it on the cigars or humidification devise before it spreads to the humidor itself. Spanish Cedar in not the same as that commonly used in closets and blanket chests.
To briefly expand on how the wood acts as a buffer, the unvarnished or stained interior of a cedar humidor takes on moisture as it is seasoned and helps to prevent spikes and troughs in RH as it takes in excess or gives off that moisture. The thicker the cedar the more stable the environment will be.
Bearing the burden of responsibility..... It's probably my fault.
Treat your silver as if it were earthenware and your earthenware as if it were silver - Seneca, Letters of a Stoic