First of all, the main usage of cDai is with the Compound protocol (smart contract protocol in DeFi). There is no other usage yet. It works as an interest from Dai lending. If we consider the price tag it is about 2 % interest. It is lower than we would expect which makes this token little bit useless. cDai is an addition to the protocol with their upgrade to v2 and exist alongside the COMP token which seems better positioned with better price tag. I would consider to use DAI as savings in other platform with higher interest.