There is alot of debate over the delivery methods and applications of roundhouse-style kicks. I practice of variety of roundhouse kicks- I started doing variations because they were effective in sparring, but I soon found other uses for many of them. Among them are:
LEADING LEG ROUNDHOUSE- a snapping kick not intended for damage, but for stunning, or set-up. The angle of execution varies from semi-downward (cutkick fashion), horizontally linear (mid-level common fashion) or upward (semi-frontkick fashion)
SKIPPING LEADING LEG ROUNDHOUSE- similar as above, but closes gap and has more impact value due to enhanced mechanics. The feinting leading leg roundhouse is similar- the kick is "faked" at low target then rebounds to hit high target (a sparring tool).
REAR LEG ROUNDHOUSE- more powerful, but still not guaranteed damage unless it hits a specific nerve center or softspot. Same angles of execution as described before. This kick rquires more setup, because it telegraphs a bit. Still somewhat a snapping kick as it is usually retracted, though it utilizes more backup mass than a basic "snap" kick.
COMMITED REAR LEG ROUNDHOUSE- Fired with full centrifugal force, full backup mass, and full commitment- the body gyrates AHEAD of the kicking leg to maximize gyroscopic energy which radiates from your torso centerline, unfurling through your hip, knee, then arch in sequential flow. Not a "quickie" strike- but a semi-finisher. You can't retract the kicking leg, it must be planted as the lead foot in the stance after execution of the kick, which limits the feasible applications of this whopper.
None of these are good for every encounter, but they each have their place in a good arsenal.