If you have multiple stage turbines there is a set of guide vanes between each turbine disc to redirect the flow again before the next turbine blade. They are stationary so are sometimes called stators.
With the turbine you want a high velocity, so turbine blades are designed to extract power from the hot expanding gasses to turn the compressor, by using the high velocity hitting the turbine blade. The guide vanes straighten the flow before the next set of turbine blades, but in effect it decreases the pressure and increases the velocity. This also prevents excessive backpressure which could work its way forward and affect the combustion process.
The compressor on the other hand is designed to increase the pressure through each of the stages, so you can deliver a good mass flow to the combustion chambers. However the increasing pressure will try to reverse the flow, so the convergent duct is designed to keep the velocity constant through the whole compressor, so that the momentum of the airflow prevents reverse flow. In effect the velocity is therefore increased in each rotor section.

Velocity increases and decreases through each set of rotors and stators, and by using a convergent duct the velocity therefore stays constant, but the pressure is steadily increasing through each stage.