Tactio can be applied where a manual measuring is taking up much time or is even impracticable.
If the tachymeter can't be placed in a suitable position, then it is difficult for the operator to determine whether a point belongs to the cross section. The advantage of Tactio is that you are free to choose the tachymeter position. It seems like measuring always from an imaginary vision point. The tachymeter doesn't have to be placed in the cross section plane.
You can adjust the measure of the details and the accuracy of the cross section. That accuracy is the same as the accuracy of the tachymeter. Besides, the set up of the measurement enables repeatability. Repetition is so accurate that deformations can be traced. You can think of a periodic measurement of a sagging building or a bending construction. The advantage to a general scan is that Tactio only measures what is relevant.