Choke Valve

"The choke is the only device used to limit the production of flowing fluids. Using a valve, such as the wing valve or master valve, to limit fluid flow would allow fluid flow (possibly with solids) to cross the sealing surface of the valve. This could lead to erosion and a leaking master valve and would require killing the well to replace the valve."

Read all...

http://gekengineering.com/Downloads/Free_Downloads/Well_Heads_Chokes_SSSV_Chapter_5.pdf

Good for being honest regarding the question.

choke or choke valve, found at the top of the Xmas tree, and as previously stated to control the flow of well fluid to the surface.
The choke is usually used with Artificial Lift, namely an ESP (Electrical Submersible Pump) to control the following

1. Flow rate to surface
2. Well head Pressure
3. Dynamic Fluid Level
4. Prevent gas breakout down hole

1. Flow rate.. an ESP has an operating range (OR) and a Best Efficiency Point (BEP), the idea is to keep the ESP operating within the OR and close to it's BEP. To do this you can open or close the choke. Then measure the RC and flow rate. Modern day down hole gauges can now tell you the flow rate, so you just adjust the choke to suit.
In an ESP you also have to be careful of upthrust in the pump caused by not keep the pump in the OR close to the BEP, where the lower pump stages rub on the pump stage above, shortening the life of the ESP. Same for downthrust.

2. Well head pressure, as you close the choke the WHP increase, flow rate goes down and so does the running current. The RC is also important to watch as you do not want to overload the motor.

3. The dynamic level, the level of fluid in the well when the ESP is running changes as you open or close the choke. Open and the DL drops and visa versa. You also need to maintain a level of fluid above the intake of the ESP, the rule of thumb is over 500ft.

4 Gas breakout or gas lock in the pump will mean you will get no fluid to surface, the ESP will trip on underload if the U/L is set correctly or you will burn the motor as there is no motor cooling. By choking back, you change the TDH thereby increasing the fluid pressure in the pump, preventing gas breaking out of solution inside the pump stages.

hope this helps...
ps.. do not over think this... it is very simple