To be the instruments of Duty, Loyalty, Honor, and Sacrifice in the Service of the Klingon Empire!
If you are looking for "roles", you actually are approaching the issue from the wrong perspective.
Klingon vessels are extremely adaptable and frankly do not easily fit into any sort of pre-conceived role. Take the Negh'var cruiser, for example...it is often used as a "tank", but it is adroit enough to mount and use cannons like an escort. The carrier has no Federation analogue - it is entirely unique in the "role" that it plays, and it takes a different kind of mindset to play it effectively. In the Federation, the "backbone" of the UFP tend to be cruisers...but in the KDF I would say that the BoP and perhaps the Vor'Cha specifically are the foundation upon which the KDF is built. KDF Ships are nimble, fast-moving, hard striking. They typically can't take a lot of damage (exceptions would be the Carriers, the T5 Orion cruiser, and possibly the Negh'Var).
KDF Ships are unique in that your playstyle can define what "role" you want to be. This is especially true of the BoP. To be candid...a Sci officer in a BoP that knows what he or she is doing is a terror to behold. They can be absolutely devastating under the right circumstances.