The discrete representation is quite enough for some applications. Scanned photos might be stored, shown to friends, changed by simple operations (cutting, turning, minimizing, etc.) even printed, if scan resolution was high enough.
The discrete representation is not enough for the other operations. First of all, a scanned text for farther usage should be made editable, for instance, in DOC or TXT format. There are some automated means of text recognition that allow to make it effectively, however, a result anyway should be checked and corrected by an operator.
Quality printing at typography requires transformation of all images and even fonts into a vector format (CorelDraw, Adobe Illustrator file and others). It is especially related to logotypes and emblems. A special feature of vector formats is mathematical correctness of contours at any scale and resolution. Besides scaling, an object of a vector format has more capabilities for other editing.
If transformed into a vector shape, an object form becomes mathematically correct. Portions become ideally smooth; rounding is being done by a part of circle, ellipse or mathematical curve. Accidental unevenness, noise, etc. are eliminated. The same happens, if discrete STL format is transformed into CAD format (IGES, STP, etc.). An object is being turned from millions of triangles into a set of cylinders, planes, smooth surfaces, etc.