system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the nozzle on a rocket engine designed to reduce the gas pressure to the external pressure ? How is it more efficient ?

I've been learning on my own how rocket engines works (well approximately), and I've understood the part about the third law of motion and conservation of momentum. But I don't understand the pressure related aspect of such engines. I've seen that depending on what altitude you mean to use it, you have to elongate the nozzle so that the pressure reduces as much as possible until at the end of the nozzle, it is equal to the external pressure. But I would think that to increase the speed of the exhaust gas, one would need  high pressure gas so that it wants to go even faster out of the engine ? 

Also I don't really understand the need for pressurizing the gas before the combustion chamber : is it to help combustion ? To reduce the size/weight of the combustion chamber ?

Thanks if you answer !
gpt
Your intuition of high pressure --> higher exhaust speed is valid, but to get the most out of that pressure you have to let it accelerate to that speed, i.e. expanding back to atmospheric pressure. Whatever pressure above atmospheric is left over at the end of the nozzle continues to accelerate the exhaust without actually providing more thrust.