A nozzle is generally used to transform total enthalpy in kinetic energy and the way one can do that depends on the pressure ratio between the absolute pressure in the reservoir and the .absolute pressure in the ambient the nozzle is exhausting (pressure ratio).

For diatomic gases if the pressure ratio is lower than about 1.9, it is convenient to use a simply convergent nozzle.

For higher pressure ratios to reach higher velocities, one may use a convergent-divergent nozzle which has an area ratio (exit/throat) depending on it.

See: Hodge & Koenig - Compressible Fluid Dynamics - Prentice Hall

The formulas reported in there are valid as long as the flow is one-dimensional, i.e. as long as the nozzle area variation is smooth.

Irrespective of applications whether subsonic or supersonic flow, the throat area has to be maintained as minimum as possible to avoid  unwanted effects in the downstream flow (possible shear layer). Increasing the throat (constant) area may cause a BL to grow which can create a secondary effect.  

Convergent nozzles are preferred for subsonic nozzle and a maximum Mach number at the throat can reach 1 with a nozzle pressure ratio of 1.89 (theoretically). When the pressure ratio increase greater than 1.89, there is a possibility of shock wave (loss in pressure) in the downstream of a nozzle due to uncontrolled expansion. This lose can be avoided using a careful design of C-D nozzle which can ensure a uniform supersonic flow at the divergent portion for a given pressure ratio.

From the above explanations, one can easily understand that the convergent nozzles are used for subsonic airplanes. Where ever supersonic flow is desired, a C-D nozzle is essential (supersonic wind tunnel, supersonic aircrafts & fighter planes etc).  

Saludos

Si la sección convergente esta unida inmediatamente a la sección divergente, la garganta de la tobera tiene longitud cero. Si la sección convergente se une con otra sección de longitud L el cual mantiene su diámetro y esta sección se une a la sección divergente, entonces la tobera tiene una garganta de longitud L (sección recta). (Este argumento es mi apreciación).

Es entendido que la garganta es la sección donde se encuentra el diámetro mínimo de una tobera convergente-divergente, donde el flujo se estrangula en la garganta cuando alcanza la velocidad del sonido, Mach 1.