Why use DTLS over SCTP when there is already TLS over SCTP ?
Is there any (good) reason or advantage for that idea?
TLS assumes an ordered delivery of cleartext messages. DTLS does not. SCTP supports both ordered and unordered delivery of messages.
So if you have an SCTP application that utilises unordered messages and needs an encryption layer then DTLS would be a good answer.
(Note however that WebRTC solves this the other way up, encapsulating the SCTP packets in DTLS and sending them over the wire.)