The primary function of a needle valve is isolation. The secondary function could be throttling (although not recommended as this can result in erosion of the sealing surface) and the third function is reduction, where the seat aperture in the needle valve is a smaller bore than the piping bore.
Needle valves are used in instrumentation systems and work on an identical principle to screwed stem globe valves. The difference being that instead of a plug point, a needle valve has, as you would expect, a needle point.
Operation is generally by a t-bar, rotated to push the needle into the seat. The seat to seal interface is the needle passing through and sealing on a conical area machined inside the body (or on a seat insert screwed / swaged into the body). Energisation of the seal is by the mechanical force applied through the stem.