These devices are independent computers used for controlling most any common 3 phase motor. Most single phase motors will not work correctly with this type of control.
Some of the benifits  are:

Soft starting - you program the ramp up to speed time. No more sudden burst of torque
and the related damage it can cause. Using a controller will also eliminate the large electrical surge that occures when ever you flip the switch to directly powered a motor.

Controlled stoping - coast to a stop, ramp to a stop, or use up to 200% of the motors rated H.P. to 
stop the machinery immeadiately.

You select the speed you want - would  20% more R.P.M. be a benifit in your application? 
Most 3 phase motors can run at twice their rated speed without penality and this control allows you to choose the speed your motor will run at. Speed selection can be made from a digital input, selector switch or infinitely variable control knob. 

Greater H.P. - Your control will sense when motor slowing occurs and then compensate by boosting tour motor's power at the instant it is needed. You can also receive fault signals if the motor performance falls outside the limits you have set.

Generate 3 phase from your single phase source - being a digital device it consumes almost no electricity at idle, it does not vibrate, and runs absolutly quiet. 

Not just for milling spindles - ever wish you could just turn a knob to speed up or slow down your lathe chuck? Or control the speed of a large drill press , fan, older manual machine tool . . . .
Most any motor that runs on 3 phase electricity can be adjusted to opperate at your desired speed.

On the complex side there are a tremendous number of other features built right into this control.. These units are shipped with a very complete manual and all of the features are available for one low price.