It is certified as both sealer AND filler.
Since the exact distance from the landmark to the apex can be determined, it is a simple matter to place a rubber stop on the needle much like one does with a file and thus position the needle accurately.
Since the only quality control method we have available is the post-operative radiograph, if there is radio-opacity in the canal, it is in fact cement.
With gutta percha it is possible to have radio-opacity with absolutely no sealer around it making it difficult to detect deficiencies in the method.
When cement is spun in with a lentulo or daubed in with a paper point, there is no control of the quantity.
When a syringe is used, by controlling the position of the needle and the rate at which the plunger is turned, quantity can be predetermined.
At any point one can backtrack and correct any errors.
Since it is so easy to correct errors with cement, most operators WILL take the time to do so.
Preparation is faster since the operator need only concentrate on getting the canal clean.
Most of the effort in preparation for gutta percha is spent getting the canal universally tapered and round througout.
Cement will flow into all canal shapes because it is amorphous and fluid.
If the filling is a bit short, the entire cement fill can be nudged apically while maintaining the seal.
In a worse case scenario, the entire contents of the canal can be removed and refilled in a few minutes.
It takes less than a minute to completely seal and fill a canal.
Cements can be premixed and syringes loaded before the appointment.
since cements are generally hard but brittle they can be removed with ultrasonics, hand files or most easily with the Fine-Cut Endo Handpiece.
In fact what is usually done is the post prep is done before filling and then the canal is filled from the apex up to the post prep.
Since much of the prep is post prep, enlargement is easier and faster as the operator has measurements readily available.
It is often faster to do both root canal prep and post prep simultaneously than to do either separately.