When saying bye, it seems slightly final but with CUL, you will see each other later and therefore, it is more conversational and friendly. CUL is marginally more articulate and less frolicsome as compared to cya and can be applied to a broader scope of informal conversations. It is use with the purpose of rejoining soon. That is why people use CUL when they do not want to end a conversation abruptly and the flow of the interaction should be more smooth. Know more about what does cul mean