The exhaust gases (and the bypass air in turbofans) are expanded and accelerated through nozzles, accelerating the flow, by reducing the static pressure of the stream, and provides propulsive force. Convergent nozzles have a progressive reduction in flow area and are used to accelerate flows to subsonic velocities. Convergent-divergent nozzle enable supersonic exit velocities provided a sufficient pressure differential exists.
The common view is that nozzles provide most of the thrust in practice there is often significant thrust provided by the pressure differentials across other components of the engine such as intakes.
A gas turbine (GT) engine or jet engine is used not only in aircraft but also in various other contrivances power generation, ships and even battle tanks (the US M1A2 Abrams tank uses a 1000 hp gas turbine). In these cases, the nozzle design isn't of much importance because the engine is not used for providing any thrust. In fact, these GT engines are replacements to the usual piston engine because of the high power to weight ratio. Hence the shaft of the GT is coupled to the required contivace, say a generator or a propeller or a gearbox.The exhaust gases from a GT engine in these scenarios get the same treatment that the exhaust gases from the piston engine.
But when the same GT engine is used in an aircraft a nozzle is used because the hot exhaust gases coming out of the GT will have very high pressure which will in turn propel the aircraft forward in accordance to Newton's 3rd Law of motion which states that "EVERY ACTION HAS AN EQUAL AND OPPOSITE REACTION" .
- Action----> Hot gases at high pressure from nozzle.
- Reaction--> The aircraft being propelled forward.
Nozzles were, until recently, only used for proving thrust but now modified nozzles are being used for changing the course of the aircraft which was traditionally done only using aileron and tail fins.