Cdk becomes partially active once its bound to cyclin and then gets phosphorylated and fully active once a Cdk-activating kinase (CAK) phosphorylates the partially active Cdk. This fully activated Cdk complex can be inactivated through a Wee1 kinase which phosphorylated the Cdk at a different site or through the binding of a Cdk Inhibitor protein (CKI).
Why would the cell need more than one way to inactivated this complex? Are there two methods in case one of them should "fail?" (i.e. the kinase not phosphorylate the Cdk complex in the proper spot) or is there some alternative reason?