I presume you are referring to a DC motor controlled by an electronic regulator. The purpose of the reactor is generally to reduce the dv/dt (rate of change of voltage) at the motor terminals which could lead to voltage breakdown of the windings. It also possible to add a reactor to reduce the motor current ripple under pulse width control.

Are you talking about the extra commutating (magnetic) wound poles often found in big machines. 20Hp and up.

These, generally known as interpoles or commutating poles, serve a totally different function. I assumed the questioner was referring to an externally connected series reactor.

The reactors in series can also aid in commutation.

Some of the older DC motors did not have a laminated frame and severe overheating occurs if you use them on rectified power instead of dc from an MG set. Series reactors can fix that problem.

Original question a little vague.